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DESCRIPTIVE  MINERALOGY 


DESCRIPTIVE 
MINERALOGY 


BY 

WILLIAM  SHIRLEY  BAYLEY,  Ph.D. 

FBOFBSSOB  OP  OEOLOOT,  UNIVBR8ITT  OF  ILLINOIS 
AUTHOR   OF    "ELEMENTABT    CRT8TALLOOBAPHT " 


WITH  TWO  HUNDBED  AND  SIXTY-EIGHT  ILLUSTRATIONS 


D.  APPLETON  AND   COMPANY 
NEW  YORK  AND  LONDON 

1917 


Copyright,  1917,  by 
D.  APPLETON  AND  COMPANY 


PRINTED  IN  THE  UNITED  STATES   OF  AMERICA 


TO 
MY  HELPER 

MY    WIFE 

THIS  BOOK  IS 
DEDICATED 


PREFACE 

The  following  pages  are  presented  with  the  purpose  of  affording 
students  a  comprehensive  view  of  modern  mineralogy  rather  than  a 
detailed  knowledge  of  many  minerals.  The  minerals  selected  for 
description  are  not  necessarily  those  that  are  most  common  nor  those 
that  occur  in  greatest  quantity.  The  list  includes  those  that  are  of 
scientific  interest  or  of  economic  importance,  and,  in  addition,  those 
that  illustrate  some  principle  employed  in  the  classification  of  minerals. 
The  volume  is  not  a  reference  book.  It  is  offered  solely  as  a  textbook. 
It  does  not  pretend  to  furnish  a  complete  discussion  of  the  mineral 
kingdom,  nor  a  means  of  determining  the  nature  of  any  mineral  that 
may  be  met  with.  The  chapters  devoted  to  the  processes  of  deter- 
minative mineralogy  are  brief,  and  the  familiar  "  key  to  the  determina- 
tion of  species  "  is  omitted.  In  place  of  the  latter  is  a  simple  guide 
to  the  descriptions  of  minerals  to  be  found  in  the  body  of  the  text. 
For  more  complete  determinative  tables  the  reader  is  referred  to  one 
of  the  many  good  books  that  are  devoted  entirely  to  this  phase  of  the 
subject.  In  the  descriptions  of  the  characteristic  crystals  of  minerals 
both  the  Naumann  and  the  Miller  systems  of  notation  are  employed, 
the  former  because  of  its  almost  general  use  in  the  more  important  refer- 
ence books  and  the  latter  because  of  its  almost  universal  use  in  modern 
crystallographic  investigations.  The  student  must  be  familiar  with 
both  notations.  It  is  thought  that  this  familiarity  can  be  best  acquired 
by  employing  the  two  notations  side  by  side. 

In  preparing  the  descriptive  matter  the  author  has  made  extensive 
use  of  Hintze's  Handbuch  der  Mineralogie.  The  figures  illustrating 
crystal  forms  are  taken  from  many  sources.     A  few  illustrations  have 

vu 


viii  PREFACE 

been  made  especially  for  this  volume.  Figures  copied  to  illustrate 
special  features  are  accredited  to  their  authors.  The  statistics  are 
mainly  from  the  Mineral  Resources  of  the  United  States.  They  are 
given  for  the  year  191 2  because  this  was  a  more  nearly  normal  year  in 
trade  than  any  that  has  followed. 

The  author  is  under  obligation  to  the  McGraw-Hill  Book  Company 
for  permission  to  reproduce  a  number  of  illustrations  originally  published 
in  his  Elements  of  Crystallography,  and  also  for  the  use  of  the  original 
engravings  in  making  the  plates  for  Figures  11,  $$t  71, 90,  no,  114,  115, 
118,  160,  191,  194,  224,  240,  and  248. 

W.  S.  Bayley. 


CONTENTS 

PART  I 
GENERAL  CHEMICAL  MINERALOGY 

CHAPTER  PAGE 

I.  The  Composition  and  Classification  of  Minerals i 

II.  The  Formation  of  Minerals  and  Their  Alterations 17 

PART  II 
DESCRIPTIVE  MINERALOGY 

III.  Introduction — The  Elements 36 

IV.  The    Sulphides,    Tellurides,    Selentdes,    Arsenides,    and 

Antimonides 68 

V.  The  Sulpho-salts  and  Sulpho-ferrites 116 

VI.  The  Chlorides,  Bromides,  Iodides,  and  Fluorides 134 

VII.  The  Oxtdes 146 

VIII.  The  Hydroxides 179 

IX.  The  Aluminates,  Ferrites,  Chromites  and  Manganttes  ...  195 

X.  The  Nitrates  and  Borates 205 

XI.  The  Carbonates 212 

XII.  The  Sulphates 236 

XIII.  The  Chromates,  Tungstates  and  Molybdates 253 

XIV.  The  Phosphates,  Arsenates  and  Vanadates 261 

XV.  The  Columbates,  Tantalates  and  Uranates 293 

XVI.  The  Silicates:  The  Anhydrous  Orthosilicates 300 

XVII.  The  Silicates:  The  Anhydrous  Metasilicates 359 

XVIII.  The  Silicates:  The  Anhydrous  Trimetasilicates 408 

XIX.  The  Silicates:  The  Anhydrous  Polysilicates 426 

XX.  The  Silicates:  The  Hydrated  Silicates 441 

XXI.  The  Silicates:  The  Titanates  and  Titanosilicates 461 

PART  III 

DETERMINATIVE     MINERALOGY 

XXII.  General  Principles  of  Blowpipe  Analysis 467 

XXIII.  Characteristic  Reactions  of  the  More  Important  Elements 

and  Acid  Radicals 483 

ix 


CONTENTS 


APPENDICES 

CHAPTER  PAGE 

I.  Guide  to  the  Descriptions  of  Minerals 495 

II.  List  of  the  More  Important  Minerals  Arranged  Accord- 

ing to  Their  Principal  Constituents 513 

III.  List   of   Minerals   Arranged   According   to  their    Crys- 

tallization   521 

IV.  List  of  Reference  Books 527 

Index 529 


LIST  OF   ILLUSTRATIONS 

FIGURE  PAGE 

i.  Sodium  fluosilicate  crystals 14 

2.  Potassium  fluosilicate  crystals 14 

3.  Cross-section  of  symmetrical  vein 21 

4.  Cross-section  of  vein  in  green  porphyry 24 

5.  Diorite  dike  cutting  granite  gneiss 26 

6.  Vein  in  Griffith  mine 27 

7.  Vein  forming  original  ore-body,  Butte,  Mont 27 

8.  Druse  of  Smithsonite 28 

9.  Geodes  containing  calcite 29 

10.  Alteration  of  olivine  into  serpentine 31 

11.  Etch  figures  in  cubic  face  of  diamond 38 

12.  Crystal  of  diamond  with  rounded  edges  and  faces 38 

13.  Octahedron  of  diamond 38 

14.  Principal  "cuts"  of  diamonds 42 

15.  Premier  diamond  mines  in  South  Africa 43 

16.  The  Cullinan  diamond 43 

17.  Gems  cut  from  Cullinan  diamond 44 

18.  The  Tiffany  diamond 44 

19.  Sulphur  crystals 47 

20.  Distorted  crystal  of  sulphur 47 

21.  Copper  crystal '. 53 

22.  Crystal  of  copper  from  Keweenaw  Point 53 

23.  Plate  of  silver  from  Coniagas  Mine,  Cobalt 57 

24.  Octahedral  skeleton  crystal  of  gold  with  etched  faces 58 

25.  Iron  meteorite , 65 

26.  Widmanstatten  figures  on  etched  surface  of  meteorite 66 

27.  Realgar  crystal 70 

28.  Stibnite  crystal 72 

29.  Galena  crystal 81 

30.  Galena  crystals 82 

31.  Chalcocite  crystal 85 

32.  Complex  chalcocite  twin 85 

33.  Tetrahedral  crystal  of  sphalerite 88 

34.  Sphalerite  crystal 88 

35.  Sphalerite  octahedron 88 

36.  Greenockite  crystal 91 

37.  Pyrrhotite  crystal 92 

XI 


xii  LIST  OF  ILLUSTRATIONS 

FIGURE  PAGE 

38.  Cinnabar  crystals 98 

39.  Group  of  pyrite  crystals  in  which  the  cube  predominates 

40.  Pyrite  crystals  on  which  0(1 1 1)  predominates 

41.  Pyrite  crystal 

42.  Group  of  pyrite  crystals 

43.  Pyrite  interpenetration  twin 

44.  Marcasite  crystal 

45.  Marcasite  crystal  with  forms  as  indicated  in  Fig.  44 

46.  Twin  of  marcasite 

47.  Spearhead  group  of  marcasite 

48.  Arsenopyrite  crystals 

49.  Crystal  of  pyrargyrite 

50.  Crystal  of  proustite 

51 .  Bournonite  crystal 

52.  Bournonite  fourling  twinned 

53.  Enargite  crystal 

54.  Stephanite  crystal 

55.  Tetrahedrite  crystal 

56.  Chalcopyrite  crystal 

57.  Chalcopyrite 

58.  Chalcopyrite  twin : 

59.  Hopper-shaped  cube  of  halite 

60.  Group  of  fluorite  crystals  from  Weardale  Co 

61.  Crystal  of  fluorite 

62.  Interpenetration  cubes  of  fluorite,  twin 

63.  Photographs  of  snow  crystals 

64.  Zincite  crystal 

65.  Hematite  crystals 

66.  Corundum  crystal 

67.  Corundum  crystal 

68.  Corundum  crystal 

69.  Quartz  crystal  exhibiting  rhombohedral  symmetry 

70.  Ideal  (A)  and  distorted  (B)  quartz  crystals 

71.  Etch  figures  on  two  quartz  crystals  of  the  same  form 

7  2.  Group  of  quartz  crystals 

73.  Tapering  quartz  crystal 

74.  Quartz  crystal 

75.  Supplementary  twins  of  quartz 

76.  Quartz  twinned 

77.  Cassiterite  crystal 

78.  Cassiterite  crystal 

79.  Cassiterite  twinned 

80.  Rutile  crystals 

81.  Rutile  eightling  twinned 


02 
02 
02 

03 

03 
10 

10 

10 

10 

12 

18 

19 
21 

21 
23 

25 

28 

31 

31 

31 

35 

39 
40 

40 

47 
SO 

53 
56 
56 
56 
59 

59 
60 

60 

61 

61 

62 

63 
69 
69 

69 

72 

72 


LIST  OF  ILLUSTRATIONS  xiii 

FIGURE  PACE 

82.  Rutile  twinned 172 

83.  Rutile  cyclic  sixling  twinned 173 

84.  Rutile  twinned 173 

85.  Anatase  crystal 177 

86.  Anatase  crystal 177 

87.  Brookite  crystals 178 

88.  Brucite  crystal 182 

89.  Limonite  stalactites  in  Silverbow  mine 184 

90.  Botryoidal  limonite 184 

91.  Pisolitic  bauxite  from  near  Rock  Run 187 

92.  Diaspore  crystals 190 

93.  Manganite  crystal 192 

94.  Group  of  prismatic  manganite  crystals 192 

95.  Manganite  crystal  twinned 193 

96.  Spinel  twin , 196 

97.  Spinel  crystal r .  196 

98.  Magnetite  crystal 198 

99.  Chrysoberyl  crystal 203 

00.  Chrysoberyl  twinned 203 

01.  Chrysoberyl  pseudohexagonal  sixling 203 

02.  Hausmannite 204 

03.  Borax  crystal 207 

04.  Colemanite  crystals 209 

05.  Boracite  crystal 211 

06.  Calcite  crystal.  * 214 

07.  Calcite  crystals 214 

08.  Calcite  crystals 214 

09.  Calcite 214 

10.  Prismatic  crystals  of  calcite 215 

11.  Calcite 215 

12.  Calcite:  twin  and  polysynthetic  trilling. 215 

13.  Calcite 216 

14.  Artificial  twin  of  calcite 216 

15.  Thin  section  of  marble  viewed  by  polarized  light.. 216 

16.  Aragonite  crystal 224 

17.  Aragonite  twin 224 

18.  Trilling  of  aragonite 224 

19.  Witherite  twinned 226 

20.  Cerussite  crystal 227 

21.  Cerussite  trilling  twinned 227 

22.  Cerussite  trilling  twinned 227 

23.  Radiate  groups  of  cerussite  on  galena 228 

24.  Dolomite  crystal 229 

25.  Group  of  dolomite  crystals 230 


XIV 


LIST  OF  ILLUSTRATIONS 


FIGURE  PAGE 

26.  Malachite  crystal 232 

27.  Azurite  crystals 233 

28.  Trona  crystal 235 

29.  Gaylussite  crystal 235 

[30.  Glauberite  crystal 237 

[31.  Thenardite  crystal 237 

[32.  Thenardite  twinned 237 

[33.  Barite  crystals 239 

[34.  Barite  crystals. 240 

[35.  Celestite  crystals 241 

:36.  Anglesite  crystal * 243 

[37.  Anglesite  crystal 243 

[38.  Anglesite  crystal 243 

[39.  Gypsum  crystals 247 

:4c  Gypsum  twinned 247 

[41.  Gypsum  twinned 248 

[42.  Epsomite  crystal 250 

[43.  Hanksite  crystal 252 

[44.  Crocoite  crystals 253 

[45.  Scheelite  crystal 255 

[46.  Scheelite  crystal 255 

47.  Wulfenite  crystal 257 

[48.  Wulfenite  crystal 257 

[49.  Wolframite  crystal 259 

50.  Monazite  crystal 264 

51.  Xenotime  crystals 265 

52.  Apatite  crystal 267 

53.  Apatite  crystal 267 

54.  Vanadinite  crystal ; 262 

55.  Skeleton  crystal  of  vanadinite 272 

56.  Amblygonite  crystal 275 

57.  Lazulite  crystals 276 

58.  Olivenite  crystal 277 

59.  Skorodite  crystal 286 

:6o.  Radiate  wavellite  on  a  rock  surface 287 

:6i.  Columbite  crystals 294 

:62.  Samarskite  crystals 297 

:63.  Olivine  crystals 303 

[64.  Wiilemite  crystal 307 

•65.  Phenacite  crystal 308 

[66.  Garnet  crystal  (natural  size) 310 

[67.  Garnet  crystals 310 

:68.  Garnet  crystal 310 

69.  Nepheline  crystal 314 


LIST  OF  ILLUSTRATIONS 


xv 


FIGURE 


PAGE 


70.  Zircon  crystals 317 

7 1 .  Zircon  twinned 317 

72.  Thorite  crystal 319 

73.  Andalusite  crystals 320 

74.  Topaz  crystals 323 

75.  Topaz  crystal 323 

76.  Topaz  crystal 324 

77.  Danburite  crystal 325 

78.  Zoisite  crystal 327 

70.  Epidote  crystal 328 

:8o.  Epidote  crystals , 328 

[81.  Chondrodite  crystal. .  . . : 333 

:82.  Datolite  crystal 334 

[83.  Staurolite  crystal 337 

84.  Staurolite  crystal  twinned 337 

[85.  Staurolite  crystal  twinned. .  .• 337 

[86.  Sodalite  interpenetration  twin  of  two  dodecahedrons 340 

[87.  Prehnite  crystal 344 

:88.  Axinite  crystal 346 

:8q.  Axinite  crystal 346 

:go.  Dioptase  crystal 347 

91.  Percussion  figure 348 

92.  Biotite  crystal 349 

93.  Biotite  twinned  about  a  plane 349 

94.  Etch  figures 356 

95.  Muscovite  crystal 356 

96.  Beryl  crystals 360 

97.  Beryl  crystals 360 

[98.  Cross-section  of  pyroxene 363 

[99.  Enstatite  crystal 366 

200.  Wollastonite  crystal 368 

201.  Augite  crystal 371 

202.  Augite  twinned 371 

203.  Interpenetration  twin  of  augite 371 

204.  Diopside  crystals 372 

205.  Hedenbergite  crystal 373 

206.  Acmite  crystal 376 

207.  Spodumene  crystal 379 

208.  Rhodonite  crystals 380 

209.  Ampibole  crystals 384 

210.  Kyanite  crystals 394 

211.  Bladed  kyanite  crystals  in  a  micaceous  quartz  schist 395 

212.  Calamine  crystals 39^ 

213.  Orthoclase  crystals 4*o 


xvi  LIST  OF  ILLUSTRATIONS 

FIGURE  PAG8 

214.  Orthoclase  crystals > 410 

215.  Carlsbad  interpenet ration  twins  of  orthoclase 410 

216.  Contact  twin  of  orthoclase  according  to  the  Carlsbad  law 410 

217.  Baveno  twins  of  orthoclase 411 

218.  Manebach  twin  of  orthoclase 411 

219.  Section  of  mirocline  viewed  between  crossed  nicols 414 

220.  Adularia  crystal 414 

221.  Albite  crystals 419 

222.  Albite  twinned 419 

223.  Albite  twinned 419 

224.  Twinning  st nations  on  cleavage  piece  of  oligoclase 420 

225.  Albite  twins  with  the  crystal  axis 420 

226.  Position  of  "rhombic  sections"  in  albite 420 

227.  Diagram  of  crystal  of  triclinic  feldspar 420 

228.  Potash-oligoclase  crystal 422 

229.  Scapolite  crystals 424 

230.  Clintonite  twinned  according  to  the  mica  law 427 

231.  Clinochlore  crystal 430 

232.  Clinochlore  twinned  according  to  mica  law 430 

233.  Clinochlore  with  same  forms  as  in  Fig.  232 430 

234.  Clinochlore  trilling  twinned  according  to  mica  law 430 

235.  Penninite  crystal 430 

236.  Penninite  crystal  twinned 430 

237.  Vesuvianite  crystals 433 

238.  Tourmaline  crystals 436 

239.  Tourmaline  crystals 436 

240.  Cooling  crystal  of  tourmaline 436 

241.  Cordierite  crystal 439 

242.  Apophyllite  crystals 444 

243.  Heulandite  crystal 447 

244.  Heulandite,  var.  beaumontite 447 

245.  Phillipsite  interpenetration  twin 448 

246.  Phillipsite 448 

247.  Harmotome  fourling  twinned  like  phillipsite 449 

248.  Sheaf -like  aggregates  of  stilbite 450 

249.  Laumontite  crystal 452 

250.  Divergent  groups  of  scolecite  crystals 453 

251.  Scolecite  crystal 453 

252.  Natrolite  crystals 454 

253.  Thomsonite  crystal 456 

254.  Chabazite  crystal 457 

255.  Chabazite  interpenetration  twin 457 

256.  Phacolite  with  same  form  as  in  Fig.  254 457 

257.  Analcite  crystal 459 


LIST  OF  ILLUSTRATIONS  xvii 

FIGURE  PAGE 

258.  Analcite  crystal 459 

259.  Ilmenite  crystal 463 

260.  Titanite  crystal 464 

261.  Titanite  crystal 464 

262.  Titanite  crystal 464 

263.  Simple  blowpipes 468 

264.  Bellows  for  use  with  blowpipe 468 

265.  Candle  flame  showing  three  mantles 470 

266.  Reducing  flame 471 

267.  Oxidizing  flame 471 

268.  Props  and  position  of  charcoal 473 


DESCRIPTIVE  MINERALOGY 


PART  I 
GENERAL  CHEMICAL  MINERALOGY 


CHAPTER  I 
THE  COMPOSITION  AND  CLASSIFICATION   OF  MINERALS 

Definition  of  Mineral. — A  mineral  is  a  definite  inorganic,  chem- 
ical compound  that  occurs  as  a  part  of  the  earth's  crust.  It  possesses 
characters  which  are  functions  of  its  composition  and  its  structure. 
Most  minerals  are  crystallized,  but  a  few  have  been  found  only  in  an 
amorphous,  colloidal  condition.  These  are  regarded  as  gels,  or  solid 
colloids. 

The  most  essential  feature  of  a  mineral  is  its  chemical  composition, 
since  upon  this  are  believed  to  be  dependent  all  its  other  properties. 

Chemical  Substances  Occurring  as  Minerals. — The  chemical 
substances  found  native  as  minerals  may  be  classed  as  elements  and 
compounds.  The  latter  comprise  chlorides,  fluorides,  sulphides,  oxides, 
hydroxides,  the  salts  of  carbonic,  sulphuric,  phosphorus,  arsenic,  anti- 
mony and  silicic  acids,  a  large  series  of  complicated  compounds  known 
as  the  sulpho-salts,  a  few  derivatives  of  certain  metallic  acids — the 
aluminates  and  the  ferrites — besides  other  salts  of  rarer  occurrence, 
some  simple  and  others  exceedingly  complicated,  and  possibly  many 
solid  solutions  of  gels  or  of  a  gel  and  a  crystalloid.  In  some  of  these 
classes  all  the  compounds  are  anhydrous.  In  others,  some  groups  are 
anhydrous  while  the  members  of  other  groups  contain  one  or  more 
molecules  of  water  of  crystallization. 

The  sulphides,  chlorides  and  fluorides  are  derivatives  of  H2S,  HC1, 
and  H2F2,  respectively.  They  may  be  regarded  as  having  been  pro- 
duced from  these  compounds  by  the  replacement  of  the  hydrogen  by 
metals.    Illustrations:    CU2S,  CuS,  NaCl,  CaF2. 


2  GENERAL  CHEMICAL  MINERALOGY 

The  hydroxides  and  the  oxides  may  be  looked  upon  as  derivatives  of 
water,  the  hydroxides  through  the  replacement  of  one  atom  of  hydrogen 
by  a  metal,  and  the  oxides  through  the  replacement  of  both  hydrogen 

/OH 

atoms.    The   mineral,   brucite,   according   to   this  view   is   Mg<Q        , 

x)H 

derived  from  rr/rkxrv  by  replacement  of  two  hydrogen  atoms  in  two 

H(0H)  Cu\ 

molecules  of  water  by  one  atom  of  Mg.    Cuprite  is        yO,  and  tenoriU 

CuO,  the  former  derived  by  replacement  of  each  atom  of  hydrogen  in 
one  molecule  of  water  by  an  atom  of  Cu,  and  the  latter  by  replacement 
of  the  two  hydrogens  by  a  single  Cu. 

The  salts  of  carbonic  acid  (H2CO3)  are  the  carbonates,  those  of  sul- 
phuric acid  (H2SO4)  the  sulphates,  those  of  orthophosphoric  acid 
(H3PO4)  the  phosphates,  those  of  orthoarsenic  acid  (H3ASO4)  the  arsen- 
ates, those  of  orthoantimonic  acid  (HaSbO-O  the  antimonates  and  those 
of  the  silicic  acids,  the  silicates.  There  are,  in  addition,  a  few  arsenites 
and  antimonites  that  are  salts  of  arsenious  (H3ASO3)  and  antimonous 
(HaSbOs)  acids. 

The  principal  silicic  acids  whose  salts  occur  as  minerals  are  normal 
silicic  acid  (H4Si04),  metasilicic  acid  (H2Si03),  and  trisilicic  acid 
(I^SiaOs).  The  metasilicic  and  the  trisilicic  acids  may  be  regarded 
as  normal  silicic  acid  from  which  water  has  been  abstracted,  in  the  same 
way  that  pyrosulphuric  acid  is  ordinary  sulphuric  acid  less  H2O,  thus: 
2H2SO4-  H20  =  H2S2O7. 

(HO)4Si— H20  =  H2Si03,  metasilicic  acid 
3(HO)4Si— 4H20=H4Si308,  trisilicic  acid. 

Fayalite  is  Fe2Si04,  wollastonite,  CaSi03,  and  orthoclase,  KAlSiaOs. 

The  aluminates  and  f errites  may  be  regarded  as  salts  of  the  hypothet- 
ical acids  AIO(OH)  and  FeO(OH),  both  of  which  exist  as  minerals, 
the  first  under  the  name  diaspore  and  the  second  under  the  name 

-AlO 


goethite.    Spinel  is  the  magnesium  aluminate,  Mg<^  ,(MgAl204), 

X)— AlO 

and  magnoferrite  the  corresponding  ferrate  MgFe204.    The  very  com- 

/O— FeO 

mon  mineral  magnetite  is  the  iron  ferrate  Fe<^  ,  or  Fe304.     In 

N>- FeO 

this  compound  the  iron  is  partly  in  the  ferrous  and  partly  in  the  ferric 
state. 


COMPOSITION  AND  CLASSIFICATION  3 

There  are  other  minerals  that  differ  from  those  of  the  classes  above 
mentioned  in  containing  more  or  less  water  of  crystallization.  These 
are  usually  separated  from  those  in  which  there  is  no  water  of  crystal- 
lization under  the  name  of  hydrous  salts. 

Besides  the  classes  of  minerals  considered  there  are  others  which 
appear  to  be  double  salts,  in  which  two  substances  that  may  exist 
independently  occur  combined  to  form  a  third  substance  with  prop- 
erties different  from  those  of  its  components.  Cryolite,  3NaF#AlF3 
or  Na^AlFe,  is  an  example.  The  sulphosalts  furnish  many  other 
examples. 

Further,  a  large  number  of  minerals  are  apparently  isomorphous 
mixtures  of  several  compounds.  These  are  homogeneous  mixtures 
of  two  or  more  substances  that  crystallize  with  the  same  sym- 
metry, and,  consequently,  that  may  crystallize  together.  Their 
physical  properties  are  continuous  functions  of  their  chemical  com- 
positions. Other  minerals  are  apparently  solid  solutions  in  one  an- 
other of  simple  crystallizable  salts,  of  gels,  of  gels  and  salts,  and  of 
gels  and  adsorbed  substances.  Among  these  are  some  of  the  commoner 
silicates. 

Determination  of  Mineral  Composition. — Since  the  properties 
of  minerals  are  functions  of  their  chemical  compositions,  it  is  important 
that  their  compositions  be  known  as  accurately  as  possible.  It  is 
necessary  in  the  first  place  that  pure  material  may  be  secured  for  study. 
Pure  material  is  most  easily  secured  by  making  use  of  the  differences 
in  density  exhibited  by  different  compounds.  The  mineral  to  be  studied 
is  pounded  to  a  powder,  sifted  through  a  bolting  cloth  sieve  and  shaken 
up  with  one  of  the  heavy  solutions  employed  in  determining  specific 
gravities.  When  the  solution  is  brought  to  the  same  density  as  that 
of  the  mineral  under  investigation  all  material  of  a  higher  specific  gravity 
will  sink.  The  material  with  a  density  lower  than  that  of  the  solu- 
tion will  rise  to  the  surface.  Material  with  a  specific  gravity  identical 
with  that  of  the  solution  will  be  suspended  in  it.  If  the  mixing  is  done 
in  a  separating  funnel  of  the  proper  type,  the  materials  may  be  drawn 
off  into  beakers  in  the  order  of  their  densities,  and  thus  the  pure  mineral 
may  be  separated  from  the  impurities  that  were  originally  incorporated 
with  it.  After  the  purity  of  the  substance  is  assured  by  examination 
under  the  microscope,  it  is  ready  for  analysis. 

The  composition  of  the  purified  material  is  determined  by  the 
ordinary  methods  of  chemistry  known  as  analysis  and  synthesis. 

In  analysis  the  compound  is  broken  into  its  constituent  parts  and 
these  are  weighed;  or  it  is  decomposed  and  its  constituents  are  trans- 


4  GENERAL  CHEMICAL  MINERALOGY 

formed  into  known  compounds  which  axe  weighed.  From  the  weights 
thus  obtained  the  proportions  of  the  components  in  the  original  sub- 
stance may  be  easily  calculated  if  the  .weight  of  the  original  substance 
be  known. 

In  synthesis  the  compound  is  built  up  from  known  elements  or 
compounds. 

If  the  mineral  calcite  (CaCOs)  is  decomposed  by  heat  into  lime 
(CaO)  and  carbonic  acid  gas  (CO2),  or  if  its  components  are  trans- 
formed into  the  known  compounds  CaS04  and  K2CO3,  the  process  is 
analysis.  If  the  known  substance  CO2  is  allowed  to  act  upon  the 
known  substance  CaO  and  the  resulting  product  is  a  substance  possess- 
ing all  the  properties  of  calcite,  the  process  is  synthesis. 

Analytical  Methods. — The  analytical  methods  made  use  of  in 
mineralogy  are:  (1)  the  ordinary  wet  methods  of  chemical  analysis, 
(2)  the  dry  methods  of  blowpipe  analysis,  in  which  the  mineral  is 
treated  before  the  blowpipe  without  the  use  of  liquid  reagents  except 
to  a  very  subordinate  degree,  and  (3)  microchemical  methods,  per- 
formed on  the  stage  of  a  compound  microscope. 

Blowpipe  and  microchemical  analyses  are  made  use  of  principally 
for  the  identification  of  minerals.  By  their  aid  the  nature  of  the  atoms 
in  a  compound  may  easily  be  learned,  but  the  proportions  in  which 
these  atoms  are  combined  is  determined  only  with  the  greatest  difficulty. 
The  methods  are  mainly  qualitative. 

Wet  Analysis. — For  exact  determinations  of  composition  the  wet 
methods  of  chemistry  are  usually  employed,  since  these  are  the  most 
accurate  ones.  They  are  identical  with  the  methods  described  in 
manuals  of  quantitative  analysis,  and  therefore  require  no  detailed 
discussion  here.  They  are  well  illustrated  by  Prof.  Tschermak  as 
follows:  If  734  mg.  of  the  mineral  goethite  (in  which  qualitative  tests 
show  the  presence  of  iron  oxide  and  water)  are  roasted  in  a  glass  tube, 
water  is  given  off.  This  when  caught  and  condensed  in  a  second  tube 
containing  dry  calcium  chloride  increases  the  weight  of  this  second 
tube  by  75  mg.  The  residue  of  the  mineral  left  in  the  first  tube  now 
weighs  about  660  mg.  An  examination  of  this  residue  shows  it  to  con- 
sist exclusively  of  the  iron  oxide  (Fe20s).  Since  only  iron  oxide  and 
water  are  present  in  goethite  the  sum  of  these  two  constituents  ought  to 
equal  the  original  weight  of  the  mineral  before  roasting.  But  660+75 
=  735,  whereas  the  original  weight  was  734.  The  difference  1  mg.  is 
due  to  unavoidable  errors  of  manipulation.  As  it  is  very  small  it  may 
be  neglected  in  our  calculations. 

The  results  of  the  analysis  are  generally  expressed  in  percentages, 


COMPOSITION  AND  CLASSIFICATION  S 

which  are  obtained  by  dividing  the  weights  of  the  different  constituents 
by  the  weight  of  the  original  substance. 


Thus:  660-5-734=89.92  per  cent  Fe2Q3 

75-5-734=10.22  per  centlfeO 


Total  100.14 

The  usual  methods  of  analysis  are,  however,  more  indirect  than  this, 
the  components  of  the  substance  to  be  analyzed  being  first  transformed 
into  known  compounds  and  then  weighed.  For  instance,  common  salt 
is  known  by  qualitative  tests  to  contain  only  Na  and  CI.  If  345  mg. 
of  the  pure  salt  be  dissolved  in  water  and  the  solution  be  treated  with 
silver  nitrate  under  proper  conditions  a  precipitate  of  silver  chloride 
is  formed  so  long  as  any  sodium  chloride  remains  in  the  solution.  The 
silver  chloride  is  separated  from  the  solution  by  nitration.  It  contains 
all  the  chloride  present  in  the  345  mg.  of  salt.  After  drying,  its  weight 
is  determined  to  be  840  mg.  The  solution  from  which  the  silver  chloride 
was  separated  contains  all  the  sodium  that  was  originally  present  in 
the  salt,  but  now  it  is  in  combination  with  nitric  acid.  It  contains 
also  any  excess  of  silver  nitrate  that  was  added  to  precipitate  the  chlorine. 

NaCl  +  AgN03  =  AgCl  +  NaN03 

salt  reagent         precipitate         filtrate 

The  nitrate  is  now  treated  with  hydrochloric  acid  to  precipitate 
the  excess  silver.  The  silver  chloride  precipitate  is  removed  by  filtra- 
tion, leaving  a  solution  containing  sodium  salts  of  nitric  and  hydro- 
chloric acids  besides  some  free  acid  of  each  kind.  Sulphuric  acid  is 
now  added  and  the  whole  solution  is  evaporated  to  dryness.  The  free 
acids  are  driven  off  by  the  heat  and  the  sodium  salts  are  transformed 
into  the  sulphate,  Na2S04.  The  residue  consisting  exclusively  of  Na2SC>4 
is  now  found  to  weigh  419  mg. 

The  345  mg.  of  salt  have  yielded  840  mg.  of  AgCl  and  419  mg.  of 
Na2S04.  The  silver  chloride  is  known  to  contain  24.74  per  cent  of 
chlorine  and  the  sodium  sulphate  32.39  per  cent  of  sodium.  The  840 
mg.  of  AgCl  contain  207.8  mg.  of  chlorine,  and  the  419  mg.  of  Na2S04 
contain  135.7  m8-  °f  sodium.    Hence  345  mg.  of  salt  yield 

207.8  mg.    or   60.23  per  cent  CI, 
and  135.7  mg-   or   39-34  per  cent  Na 

343.5  mg.         99.57  per  cent 


GENERAL  CHEMICAL  MINERALOGY 


Records  of  Analyses. — The  composition  of  minerals  like  that  of 
other  chemical  compounds  is  determined  in  percentages  of  their  com- 
ponents and  is  recorded  as  parts  per  ioo  by  weight.  A  weighed  quantity 
of  the  mineral  is  analyzed,  the  products  of  the  analysis  are  weighed  and  the 
percentage  of  each  constituent  present  is  found  by  dividing  its  weight 
by  the  weight  of  the  original  substance,  as  has  already  been  indicated. 

In  chemical  treatises  the  results  of  the  analyses  are  usually  recorded 
in  percentages  of  the  elements  present.  In  mineralogical  works  it  is 
more  common  to  write  the  percentage  composition  in  terms  of  the 
oxides  of  the  elements,  partly  because  the  old  analyses  are  recorded  in 
this  way  and  partly  because  certain  relations  between  the  mineral 
components  can  be  better  exhibited  by  comparison  of  the  oxides  than 
by  comparison  of  the  elements  present  in  them. 

The  record  of  the  analysis  of  a  magnesik  may  be  given  as: 


28.35  per  cent, 

.34  per  cent, 

14.25  per  cent, 

56.98  per  cent, 

Total =99.92  per  cent. 


Mg 

Fe 

C 

O 


-  or  as 


MgO=47.25  per  cent, 
FeO=  .43  per  cent, 
C02= 52.24  per  cent, 

Total =99.92  per  cent. 


Calculation  of  Formulas. — After  the  determination  of  the  per* 
centage  composition  of  a  mineral,  the  next  step  is  to  represent  this 
composition  by  a  chemical  formula — a  symbol  which  indicates  the 
relative  number  of  elementary  atoms  in  the  mineral's  molecule,  instead 
of  the  number  of  parts  of  its  constituents  in  100  parts  of  its  sub- 
stance. 

The  construction  of  a  formula  from  the  analytical  results  is  simple 
enough  in  principle,  but  in  practice  it  is  often  made  difficult  by  the 
fact  that  many  apparently  pure  substances  are  in  reality  composed  of 
several  distinct  compounds  so  intimately  intercrystallized  that  it  is 
impossible  to  separate  them.  In  the  simplest  cases  the  formula  is 
derived  directly  from  the  results  of  the  analyses  by  a  mere  process  of 
division. 

The  atomic  weights  of  the  chemical  elements  are  the  relative  weights 
of  the  smallest  quantities  that  may  enter  into  chemical  combination  with 
one  another,  measured  in  terms  of  the  atomic  weight  of  hydrogen  which 
is  taken  as  unity,  or  of  oxygen  taken  as  16.  Thus  the  atomic  weights 
of  nitrogen  and  oxygen  are  approximately  14  and  16  respectively,  i.e., 
the  smallest  quantities  of  nitrogen  and  oxygen  that  can  enter  into  com- 
bination with  each  other  and  with  hydrogen  are  in  the  ratio  of  the 


COMPOSITION  AND  CLASSIFICATION 


TABLE  OF  ATOMIC  WEIGHTS 


Element  Symbol 

Aluminium Al 

Antimony Sb 

Argon A 

Arsenic As 

Barium Ba 

Bismuth Bi 

Boron B 

Bromine Br 

Cadmium Cd 

Caesium Cs 

Calcium Ca 

Carbon C 

Cerium Ce 

Chlorine CI 

Chromium Cr 

Cobalt Co 

Columbium Cb 

Copper Cu 

Dysprosium Dy 

Erbium Er 

Europium Eu 

Fluorine F 

Gadolinium Gd 

Gallium Ga 

Germanium Ge 

Glucinum Gl 

Gold Au 

Helium He 

Holmium Ho 

Hydrogen H 

Indium In 

Iodine I 

Iridium,  v Ir 

Iron Fe 

Krypton Kr 

Lanthanum La 

Lead Pb 

Lithium Li 

Lutecium Lu 

Magnesium Mg 

Manganese Mn 

Mcjcury Bg 


At.  Weight 

27.1 

120.2 

39.88 

7406 

137.37 
208.0 

11. o 

70.02 
112.40 

132-81  ' 

40.07 

12.005 

140.25 

35.46 
52.0 

58.97 

93  5 

63  57 
162.5 
167.7 
1520 

19.0 

157.3 
69.9 

72.5 
01 
197.2 
4.00 

1635 
1.008 

114. 8 

126.92 

193. 1 

5585 
82.92 

139.0 

207 . 20 

6.94 

I75-0 
24.32 

5493 
200.6 


Element  Symbol 

Molybdenum Mo 

Neodymium Nd 

Neon Ne 

Nickel Ni 

Niton Nt 

Nitrogen N 

Osmium Os 

Oxygen O 

Palladium Pd 

Phosphorus P 

Platinum Pt 

Potassium K 

Praseodymium Pr 

Radium Ra 

Rhodium Rh 

Rubidium Rb 

Ruthenium Ru 

Samarium Sa 

Scandium Sc 

Selenium Se 

Silicon Si 

Silver Ag 

Sodium Na 

Strontium Sr 

Sulphur S 

Tantalum Ta 

Tellurium Te 

Terbium Tb 

Thallium Tl 

Thorium Th 

Thulium Tm 


Tin. 


Sn 


Titanium Ti 

Tungsten W 

Uranium U 

Vanadium V 

Xenon Xe 

Ytterbium  (Neoytterbium) . .  Yb 

Yttrium Y 

Zinc Zn 

Zirconium Zr 


At.  Weight 

96.0 

144  3 
20.2 

58.68 
222.4 

14  01 
190.9 

16.0 
106.7 

3104 
195  2 

39  10 
140.9 
226.0 
102.9 

8545 
101.7 

1504 

441 

792 

28.3 
107.88 

230 

8763 
3206 
181. 5 

1275 
1592 
204.0 
232.4 
168.5 
118. 7 

48.1 
184.0 
238.2 

5106 
1302 

173-5 
88.7 

6537 
90.6 


8  GENERAL  CHEMICAL  MINERALOGY 

values  14  :  16  :  i.1  The  quantities  that  possess  these  relative  weights 
are  known  as  atoms.  Often  the  apparent  ratios  of  the  elements  in 
combination  are  different  from  the  ratios  between  their  atomic  weights, 
but  this  is  always  due  to  the  fact  that  one  or  the  other  of  the  elements 
is  present  in  more  than  its  smallest  possible  quantity,  i.e.,  in  a  greater 
amount  than  is  represented  by  a  single  atom.  For  instance,  there  are 
several  compounds  of  oxygen  and  nitrogen  known,  in  which  the  weight 
relations  between  the  two  elements  may  be  represented  by  the  follow- 
ing figures:  14  :  8;  14  :  16;  14  :  24;  14  :  32,  and  14  :  40.  If  the 
second  of  these  compounds  consists  of  one  atom  each  of  nitrogen  and 
oxygen,  and  these  are  the  smallest  quantities  of  the  elements  that 
can  exist  in  combination,  the  several  compounds  must  be  made  up  thus: 

14  :  8  14  :  16  14  :  24  14  :  32  14  :  40 

N20  NO  N2O3  N02  N2O5 

for  N  can  exist  only  in  quantities  that  weigh  14,  28,  42  times  as  much 
as  the  smallest  quantity  of  hydrogen  present  in  any  compound,  i.e., 
the  single  atom,  and  O  in  quantities  of  16,  32,  48,  etc.,  times  the  weight 
of  the  single  hydrogen  atom.  In  order  that  even  multiples  of  14  and 
16  shall  exist  in  the  ratios  given  above,  their  terms  must  be  multi- 
plied by  quantities  that  will  yield  the  following  results; 

28  :  16  14  :  16  28  :  48  14  :  32  28  :  80 

which  are  the  weights  respectively  of  the  numbers  of  atoms  represented 
in  the  above  formulas. 

If,  then,  the  elements  combine  in  the  ratio  of  their  atomic  weights, 
or  in  some  multiple  of  this  ratio,  the  figures  obtained  by  analysis  must 
be  in  one  of  these  ratios,  and  consequently  they  furnish  the  data  from 
which  the  formula  of  the  substance  analyzed  may  be  deduced.  In 
gold  chloride,  for  example,  analysis  shows  the  presence  of  64.87  per  cent 
Au  and  35.13  per  cent  CI,  i.e.,  the  gold  and  the  chlorine  are  united  in 

the  ratio  of  64.87  ;  35.13  or  .    The  combining  ratio  of  single 

00"    o 

atoms  of  gold  and  of  chlorine  is,  however,  196.7  :  35.5,  or  .     Evi- 

35-5 
dently  in  gold  chloride  the  ratio  of  gold  to  chlorine  is  only  one-third 

as  great  as  is  the  ratio  between  the  atomic  weights  of  these  elements, 

or  the  ratio  of  the  chlorine  to  the  gold  three  times  as  great.    Hence 

1  The  atomic  weight  of  hydrogen  is  more  accurately  1.008,  when  that  of  oxygen 
is  taken  as  16. 


COMPOSITION  AND  CLASSIFICATION 


0 


there  must  be  three  times  as  much  chlorine  in  gold  chloride  as  would 
be  represented  by  a  single  atom  of  chlorine,  or  there  must  be  three 
atoms  of  chlorine  in  the  compound,  for  we  cannot  imagine  a  quantity 
of  gold  present  which  is  equivalent  to  one-third  of  an  atom  of  gold. 
Gold  chloride  is  therefore  AuCU. 

We  can  now  prove  our  conclusion  by  calculation.  One  atom  of 
gold  and  three  atoms  of  chlorine  ought  to  combine  in  the  ratio  of 
196.7  :  106.5  (i.e.,  35.5X3).  If  our  conclusion  is  correct,  and  the 
gold  chloride  analyzed  is  AuCfe,  then  the  quantities  of  gold  and  of 
chlorine  yielded  by  the  analysis  should  be  in  this  ratio.  The  figures 
obtained  are  in  the  ratio  of  64.87  :  35.13.  Multiplying  both  terms  of 
this  ratio  by  3.031  we  obtain  196.62  :  106.5,  which  is  approximately 
the  ratio  expected. 

In  practice,  the  same  result  as  that  outlined  above  is  reached  by 
dividing  the  results  of  analyses  by  the  atomic  weights  of  the  various 
elements  or  groups  of  elements  concerned.  The  quotients  represent  the 
proportional  numbers  of  the  elements  or  groups  present.  If  the  small- 
est quotient  is  assumed  as  unity,  the  ratios  existing  between  this  and 
the  other  quotients  indicate  the  number  of  atoms  or  groups  of 
atoms  represented  by  the  latter. 

Illustrations; 


Gold  Chloride  Result  of  Analysis  Atomic  Weights  Quotients 

Au     =     64.87  per  cent      -5-       196.7      =       329& 
CI       -     3S-J3  ■*■        35-5      =       9896 


Ratios 
I 

3-00+ 


Tin  Chloride 

Sn      =     45.26  per  cent      -s-       117. 4     —        .384 
CI       =     54- 74  -*-         355      -      i-54* 


1 

4.04 


The  formula  of  the  gold  chloride  is  AuCk,  and  of  the  tin  chloride, 
SnCU. 

Magnesium  carbonate  on  analysis  may  yield :  C  =  14. 26 ;  Mg  =28.37; 
Fe=.34;  O—  57.03;  or,  if  recorded  in  the  form  of  oxides:  002=52.24; 
MgO= 47.25;  FeO=.43-  From  either  of  these  results  the  formula  is 
easily  obtained  by  the  method  described. 


0=14.26-5-11.97  =  1.188=1.009; 
Mg=  28.37-^23.94=  1.186=  1.000; 
Fe=     .34-*- 55.88=  .006=  .006; 
O-57.03-s-15.96-3.573-3.012;  - 


or, 


MgC03,  if  we  neglect   the  small 
quantity  of  iron  present. 


10 


GENERAL  CHEMICAL  MINERALOGY 


From  the  second  set  of  figures  we  have: 


C02= 52.244-43.89=  1.19=1; 

MgO=47-2S-5"39-90=  1.184=1; 
FeO=     .434-71.84=  .006; 


or. 


MgO-C02,    which    is    the  same  as 
MgC03,  written  in  a  different  way. 


All  formulas  are  derived  by  methods  like  these,  but  in  many  cases 
the  processes  are  made  more  difficult  by  the  impossibility  of  deciding 
positively  whether  those  substances  that  are  present  in  small  quantities 
are  present  as  impurities  or  whether  they  exist  as  essential  parts  of 
the  compound. 

Formulas  of  Substances  Containing  Two  or  More  Metallic 
Elements  or  Acid  Groups. — In  the  illustration  given  above  the  com- 
pounds consist  of  but  one  kind  of  metallic  element  combined  with  one 
kind  of  acid.  Often  in  the  case  of  minerals  there  are  present  two  or 
more  metallic  elements,  and  less  commonly  several  acid  groups.  When 
two  metals  are  present  in  definite  atomic  proportions  the  formula  is 
written  in  the  usual  manner,  as  CaMg(C03)2  for  the  mineral  dolomite, 
in  which  calcium  and  magnesium  are  present  in  the  ratio  of  one  atom 
of  each  to  two  parts  of  the  acid  group  CO3.  Very  often,  and  perhaps  in 
the  majority  of  cases,  when  two  or  more  metallic  elements  are  present 
in  different  specimens  of  a  mineral  they  are  not  found  always  in  the 
same  proportion — the  mineral  may  consist  of  isomorphic  mixtures 
of  several  substances.  For  instance,  many  calcium-magnesium  car- 
bonates are  known  in  which  the  ratio  of  calcium  to  magnesium  present 
is  not  as  1  atom  to  1  atom,  but  in  which  this  ratio  is  as  2  atoms 
to  1  atom,  3  atoms  to  2  atoms,  or  a  ratio  which  would  have  to  be 
represented  by  irrational  figures  like  2.7236  atoms  to  1.5973  atoms. 
Each  one  of  these  compounds  properly  requires  a  separate  formula, 
as  2CaC03+MgC03,  3CaC03+2MgC03,  etc.,  but  practically  the  entire 
series  of  compounds  is  represented  by  a  single  symbol,  thus:  (Ca  •  Mg)C03, 
indicating  that  in  the  series  we  have  to  do  with  mixtures  of  carbonates 
of  calcium  and  magnesium,  or  with  complex  molecules  containing  in 
different  instances  different  proportions  of  the  two  carbonates.  For 
greater  definiteness  the  symbol  of  the  characteristic  element  of  the 
substance  which  is  in  largest  quantity  in  the  compound  is  usually  written 
first,  as  (Ca-Mg)CC>3,  when  calcium  carbonate  is  in  excess,  or 
(Mg-  Ca)CC>3  when  magnesium  carbonate  predominates.  If  still  greater 
definiteness  is  desired  small  figures  are  placed  below  the  symbols  of  the 
elements  concerned,  as  (Ca2*Mgi)C03  or  (Ca3  •  Mg2)CC>3,  to  indicate 
the  respective  proportions  present.     (Ca2-Mgi)C03  signifies  that  the 


COMPOSITION  AND  CLASSIFICATION  11 

mineral  thus  represented  contains  calcium  and  magnesium  in  the 
ratio  of  2  atoms  of  the  former  to  1  of  the  latter. 

Compounds  Containing  Water. — Often  salts  that  separate  from 
aqueous  solutions  combine  with  certain  definite  proportions  of  water. 
Sometimes  this  water  combines  with  the  anhydrous  portion  of  the  com- 
pound to  form  a  double  salt,  as  MgS04+7HaO,  or  MgSO-i-ylfeO. 
At  other  times  a  portion  of  the  water,  in  the  form  of  the  group  (OH), 
called  the  hydroxyl  group,  occupies  the  place  usually  occupied  by 
a  metallic  element,  and,  occasionally,  that  usually  occupied  by  an 
acid  group,  or  by  oxygen,  as  in  Mg(OH)2. 

Water  of  Crystallization. — Double  salts  composed  of  an  anhydrous 
portion  combined  with  water  are  usually  well  crystallized.  Although 
the  water  appears  in  many  cases  to  be  but  loosely  combined  with  the 
remainder  of  the  compound  it  is  an  essential  part  of  its  crystal  particle, 
for  by  the  loss  of  even  a  portion  of  it  the  crystal  system  of  the  compound 
is  often  changed.  Water  in  this  form  is  known  as  water  of  crystalliza- 
tion, and  the  compounds  are  designated  hydrates. 

The  magnesium  sulphate  MgS04  •  7H2O  forms  orthorhombic  crystals. 
By  evaporation  of  a  hot  solution  of  this  substance  the  sulphate 
MgSO*  -  6H2O  separates  as  monoclinic  crystals. 

Gypsum  is  CaS04  •  2H2O.  Its  crystallization  is  monoclinic.  When 
heated  to  2000  it  passes  into  the  anhydrous  orthorhombic  mineral 
anhydrite,  CaS04. 

Water  of  crystallization  may  frequently  be  driven  from  the  com- 
pound in  which  it  exists  by  continued  heating  at  a  comparatively  low 
temperature.  It  is  usually  given  off  gradually — an  increase  in  the  tem- 
perature causing  an  increase  in  the  quantity  of  water  released  until 
finally  the  last  trace  disappears.  In  many  instances  such  a  very  high 
temperature  is  required  to  drive  off  the  last  traces  of  the  water  that  it 
would  appear  that  some  of  it  is  held  in  combination  in  a  different 
manner  from  that  in  which  the  remainder  is  held.  Indeed,  it  is  not  at 
all  certain  that  double  salts  containing  water  of  crystallization  are 
different  in  any  essential  respect  from  ordinary  atomic  molecules  in 
which  hydrogen  and  oxygen  are  present  in  atomic  form. 

Combined  Water. — Water  of  crystallization  is  thought  of  as 
existing  in  the  compound  as  water  because  of  the  ease  with  which  it 
can  be  driven  off.  Compounds  in  which  the  hydroxyl  group  is  present 
yield  water  only  upon  being  heated  to  comparatively  high  temperatures. 
In  them  the  elements  of  water  are  present,  but  not  united  as  water. 
When  freed  from  their  combinations  with  the  other  constituents  of  the 
compound  by  heat  they  unite  to  form  water.    Because  its  elements 


12  GENERAL  CHEMICAL  MINERALOGY 

are  thought  of  as  closely  combined  with  the  other  elements  in  the 
molecule,  this  kind  of  water  is  often  distinguished  from  water  of  crystal- 
lization by  the  term  combined  water. 

Brucile  (Mg(OH)2)  and  malachite  (Cu2(OH)2CC>3)  are  minerals 
containing  the  elements  of  water.  When  heated  they  yield  water 
according  to  the  reactions  Mg(OH)2=MgO+H20  and  Cu2(OH)2COs 
=  CuO+CuC03+H20. 

Combined  water  is  not  only  more  difficult  to  separate  from  its  com- 
bination than  is  water  of  crystallization,  but  when  the  combination 
is  broken  the  chemical  character  of  the  original  substance  is  radically 
changed,  as  may  be  seen  from  the  reactions  above  indicated.  More- 
over, combined  water  is  given  off  suddenly,  at  a  certain  minimum 
temperature,  and  not  gradually  as  in  the  case  of  water  of  crystal- 
lization. 

Blowpipe  Analysis. — Although  blowpipe  analysis  serves  merely  to 
identify  the  chemical  components  of  minerals,  it  is  a  most  important 
aid  to  mineralogists  in  their  practical  work. 

Nearly  all  minerals  may  be  recognized  with  a  close  degree  of  accu- 
racy by  their  morphological  and  physical  properties.  To  distinguish 
between  several  minerals  that  are  nearly  alike  in  these  characteristics, 
however,  the  determination  of  composition  is  often  important.  In 
cases  of  this  kind  a  single  test  made  with  the  blowpipe  will  frequently 
give  the  desired  information  as  to  the  nature  of  some  one  or  more  of 
the  chemical  elements  present,  and  thus  in  a  few  moments  the  mineral 
may  be  identified  beyond  mistake. 

The  apparatus  necessary  to  perform  blowpipe  analysis  is  very 
simple  and  the  number  of  pieces  few.  These,  together  with  all  the 
reagents  in  sufficient  quantity  to  determine  the  composition  of  hundreds 
of  minerals,  may  be  packed  into  a  box  no  larger  than  a  common  lunch 
box.     (See  pp.  467-470.) 

For  more  refined  work  than  the  mere  testing  of  minerals  a  larger 
collection  of  both  apparatus  and  reagents  is  necessary,  but  in  no  case 
is  the  quantity  of  material  consumed  in  blowpipe  analysis  as  great  as 
when  wet  methods  of  analysis  are  used. 

Principles  Underlying  Blowpipe  Analysis.— The  principal  phe- 
nomena that  are  the  basis  of  blowpipe  work  are  the  simple  ones  known 
in  chemistry  as  volatilization,  reduction,  oxidation,  and  solution. 

For  volatilization  experiments  charcoal  sticks  and  glass  tubes  are 
used.  A  blowpipe  serves  to  direct  a  hot  blast  upon  the  assay.  The 
volatilized  products  collect  on  the  cool  parts  of  the  charcoal  which 
they  coat  with  a  characteristic  color,  or  upon  the  cooler  portions  of 


COMPOSITION  AND  CLASSIFICATION  13 

the  glass  tubes.  The  sublimates  that  collect  in  the  tubes  may  be  tested 
with  reagents  or  examined  under  the  microscope. 

Some  volatile  substances  impart  a  distinct  and  characteristic  color 
to  an  otherwise  colorless  flame.  These  may  be  tested  in  the  direct  flame 
of  the  blowpipe. 

Oxidation  and  reduction  experiments  are  usually  performed  either 
on  charcoal  or  in  glass  tubes.  Oxidations  are  effected  in  open  tubes 
and  reductions  in  those  closed  at  one  end.  The  products  of  the  oxida- 
tion or  of  the  reduction  are  studied  and  from  their  characteristics  the 
nature  of  the  original  substance  is  inferred. 

The  solution  of  bodies  to  be  tested  is  often  made  in  the  usual  man- 
ner, i.e.,  by  treating  them  with  liquid  reagents,  but  more  frequently 
it  is  accomplished  by  fusion  of  a  small  quantity  of  the  body  with  borax 
(Na2B407  •  10H2O)  or  miopcosmic  salt  ((NH^NaHPO^-tffeO).  The 
molten  reagent  dissolves  a  portion  of  the  substance  to  be  tested  and  in 
many  cases  forms  with  it  a  colored  mass.  From  the  color  of  the  mass 
the  nature  of  the  coloring  matter  may  be  learned. 

Although  the  underlying  principles  of  blowpipe  analysis  are  simple 
the  reactions  that  take  place  between  the  reagents  and  the  assay  are 
often  very  complex. 

More  explicit  details  of  the  operations  of  qualitative  blowpipe 
analysis  are  given  in  Part  III. 

Microchemical  Analysis. — The  processes  of  microchemical  analysis 
are  limited  in  their  application  to  the  detection  of  a  single  element  or, 
at  most,  of  a  very  few  elements  in  small  quantities  of  minerals.  They 
are  employed  mainly  in  deciding  upon  the  composition  of  a  substance 
whose  nature  is  suspected. 

The  principle  at  the  basis  of  all  microchemical  methods  is  the  manu- 
facture of  crystallized  precipitates  by  treatment  of  the  mineral  under 
investigation  with  some  reagent,  and  the  identification  of  these  pre- 
cipitates through  their  optical  and  morphological  properties. 

In  practice,  a  small  particle  of  the  mineral  the  nature  of  which  it 
is  desired  to  know  is  placed  on  a  small  glass  plate,  which  may  be  covered 
with  a  thin  film  of  Canada  balsam  to  prevent  corrosion,  and  is 
moistened  with  a  drop  or  two  of  some  reagent  that  will  decompose 
it.  The  solution  thus  formed  is  slowly  evaporated  by  exposure  to  the 
air.  The  plate  is  then  placed  beneath  the  objective  of  a  microscope 
and  the  crystals  formed  during  the  evaporation  are  investigated.  Or, 
after  a  solution  of  the  assay  is  obtained  there  is  added  a  small  quantity 
of  some  reagent  and  the  resulting  precipitate  is  studied  under  the 
microscope.    By  their  shapes  and  optical  properties  the  nature  of  the 


GENERAL  CHEMICAL  MINERALOGY 


Fie.  i. — Sodium  Fluosilicate  Crystals.     Magnified  73 


(After  Rascnbusch.) 


—Potassium  Fluosilicate  Crystals.     Magnified  140  diam.     (After  Roienbusck.) 


COMPOSITION  AND  CLASSIFICATION  15 

crystals  produced  is  determined,  and  in  this  way  the  nature  of  the  con- 
stituehts  they  have  obtained  from  the  mineral  particles  is  discovered. 

A  large  number  of  reagents  have  been  used  in  microchemical  tests 
each  of  which  is  best  suited  to  some  particular  condition.  The  most 
generally  useful  one  is  hydrofluosilicic  acid  (H2SiFo).  If  small  frag- 
ments of  albite  and  of  orthoclase  are  placed  on  separate  glass  slips,  such 
as  are  used  for  mounting  microscopic  objects,  and  each  is  treated  with 
a  drop  of  this  reagent  and  then  allowed  to  remain  in  contact  with  the 
air  for  a  few  minutes  until  the  solutions  begin  to  evaporate,  those 
portions  of  the  solutions  remaining  will  be  discovered  to  be  filled  with 
little  crystals.  The  crystals  in  the  solution  surrounding  the  albite  are 
hexagonal  in  habit  (Fig.  i),  while  those  in  the  solution  surrounding 
the  orthoclase  are  cubes,  octahedrons  or  combinations  of  forms  belonging 
to  the  isometric  system  (Fig.  2).  The  former  are  crystals  of  sodium 
fluosilicate  and  the  latter  crystals  of  the  corresponding  potassium  salt. 
The  albite,  consequently,  is  a  sodium  compound  and  the  orthoclase  a 
compound  of  potassium.  In  similar  manner,  by  means  of  this  or  of 
other  reagents  the  constituents  of  many  minerals  may  be  easily  detected. 
The  method,  however,  is  made  use  of  only  in  special  cases,  when  for 
some  reason  or  other  analytical  methods  are  not  applicable. 

Synthesis. — Synthesis  is  the  opposite  of  analysis.  By  the  analytical 
processes  compounds  are  torn  apart,  or  broken  down,  whereas  by  syn- 
thetical operations  they  are  put  together  or  built  up.  Synthetic  methods 
are  employed  principally  in  the  study  of  the  constitution  of  minerals 
and  of  their  mode  of  formation,  and  in  the  investigation  of  the  condi- 
tions that  determine  the  different  crystal  habits  of  the  same  mineral. 
The  products  of  synthetic  reactions  are  often  spoken  of  as  artificial 
minerals  because  made  through  man's  agency.  In  many  instances 
these  artificial  minerals  are  identical  in  every  sense  with  natural  minerals. 
Consequently,  they  may  often  serve  as  material  for  study,  when  the 
quantity  of  the  natural  mineral  obtainable  is  too  small  for  the  purpose. 

Classification  of  Minerals. — Classification  is  the  grouping  of 
objects  or  phenomena  in  such  a  manner  as  will  bring  together  those 
that  are  related  or  that  are  similar  in  many  respects  and  will  separate 
those  that  are  different. 

Since  minerals  are  chemical  compounds  whose  properties  depend  upon 
their  compositions,  their  most  logical  classification  must  be  based  upon 
chemical  relationships.  But  their  morphological  and  physical  properties 
are  their  most  noticeable  features,  and  hence  these  should  also  be  taken 
into  account  in  any  classification  that  may  be  adopted.  Probably 
the  most  satisfactory  method  of  classifying  minerals  is  to  group  them, 


16  GENERAL  CHEMICAL  MINERALOGY 

first,  in  accordance  with  their  chemical  relationships  and,  second,  in 
accordance  with  their  morphological  and  physical  properties. 

The  first  division  is  into  the  great  chemical  groups,  as,  for  instance, 
the  elements,  the  chlorides,  the  sulphides,  etc.  The  second  division 
is  the  separation  of  these  great  groups  into  smaller  ones  comprising 
minerals  possessing  the  same  general  morphological  features.  These 
smaller  groups  may  contain  only  a  single  mineral  or  they  may  contain 
a  large  number  of  closely  allied  ones.  If  the  basis  of  the  subgrouping 
is  manner  of  crystallization,  it  follows  that  the  members  of  subgroups 
containing  more  than  one  member  are  usually  isomorphous  compounds. 
Thus  the  subdivisions  of  the  great  chemical  groups  are  single  minerals 
and  small  or  large  isomorphous  groups  of  minerals,  arranged  in  the 
order  in  which  their  metallic  elements  are  usually  discussed  in  treatises 
on  chemistry.  For  example,  the  great  group  of  carbonates  embraces 
all  minerals  that  are  salts  of  carbonic  acid  (H2CQ3).  This  great  group 
is  divided  into  smaller  groups  along  chemical  lines,  as  for  instance,  the 
normal  carbonates,  the  hydrous  carbonates,  the  basic  carbonates,  etc. 
These  smaller  groups  are  finally  divided  into  subgroups  according  to 
their  morphological  properties — the  normal  salts,  for  example,  being 
divided  into  the  two  isomorphous  groups  known  as  the  calcite  and  the 
aragonite  groups,  and  a  third  group  comprising  but  a  single  mineral. 

In  certain  specific  cases  some  other  classification  than  the  one 
outlined  above  may  be  desirable.  For  instance,  in  books  written  for 
mining  students  it  is  often  found  that  a  classification  based  upon  the 
nature  of  the  metallic  constituent  is  of  more  interest  than  the  more 
strictly  scientific  one  outlined  above,  because  such  a  classification 
emphasizes  those  components  of  the  minerals  with  which  the  mining 
student  is  most  concerned.  In  books  written  for  the  student  of  rocks, 
on  the  other  hand,  the  most  important  determinative  features  of  minerals 
are  their  morphological  characters,  hence  in  these  the  classification 
may  be  based  primarily  on  manner  of  crystallization. 

In  the  present  volume  the  classification  first  outlined  is  used,  but 
because  such  a  small  proportion  of  the  known  minerals  are  discussed 
the  beauties  of  the  classification  are  not  as  apparent  as  they  would  be 
were  all  described. 


CHAPTER  II 

THE  FORMATION  OF  MINERALS  AND  THEIR  ALTERATIONS 

The  Origin  of  Minerals. — Minerals,  like  other  terrestrial  chemical 
compounds,  are  the  result  of  reactions  between  chemical  substances 
existing  upon  the  earth.  When  they  are  the  direct  result  of  the  action 
of  elements  or  compounds  not  already  existing  as  minerals  they  are  said 
to  be  primary  products;  when  formed  by  the  action  of  chemical  agents 
upon  minerals  already  existing  they  are  often  spoken  of  as  secondary, 
though  this  distinction  of  terms  is  not  always  applied. 

Quartz  (SiC>2),  formed  by  the  cooling  of  a  molten  magma,  is  primary; 
when  formed  by  the  action  of  water  upon  the  siliceous  constituents  of 
rocks  it  is  secondary. 

The  Formation  of  Primary  Minerals. — Minerals  are  produced  in  a 
great  variety  of  ways  under  a  great  variety  of  conditions.  Even  the 
same  mineral  may  be  produced  by  many  different  methods.  The  more 
common  methods  by  which  primary  minerals  are  formed  are:  precipita- 
tion from  a  gas  or  a  mixture  of  gases,  precipitation  from  solution,  the 
cooling  of  a  molten  magma,  and  abstraction  from  water  or  air  by  plants 
and  animals. 

Deposits  from  Gases. — Emanations  of  gases  are  common  in  vol- 
canic districts.  The  gases  escaping  from  volcanic  vents  are  mainly 
water  vapor,  hydrochloric  acid,  sulphur  dioxide,  sulphuretted  hydro- 
gen, ammonia  salts  and  carbon  dioxide,  besides  small  quantities  of  other 
gases  and  the  vapors  of  various  metallic  compounds.  By  the  reactions 
of  these  with  one  another  or  with  the  oxygen  of  the  air,  sulphur,  salam- 
moniac  (NH4CI)  and  other  substances  may  be  formed,  and  by  their 
reaction  upon  the  rocks  in  the  neighborhood  halite  (NaCl),  ferric  chlo- 
ride (FeCla),  hematite  (Fe203)  and  many  other  compounds  may  be 
produced. 

The  production  of  minerals  through  the  reactions  set  up  between 
various  gases  and  vapors  is  known  as  pneutnatolysis.  Their  separation 
from  the  gaseous  condition  is  known  as  sublimation.  Minerals  formed 
by  sublimation  are  usually  deposited  as  small,  brilliant  crystals  on  the 
surfaces  of  rocks  or  upon  the  walls  of  cavities  and  crevices  in  them. 

17 


18  GENERAL  CHEMICAL  MINERALOGY 

The  reactions  by  which  they  are  produced  are  often  quite  simple.  Thus 
the  reaction  between  sulphuretted  hydrogen  and  sulphur  dioxide  yields 
sulphur  (2H2S+S02=3S+2H20),  as  does  also  the  reaction  between  the 
first  named  gas  and  the  oxygen  of  the  atmosphere  (H2S+0=H20+S). 
Ferric  chloride  may  be  produced  by  the  action  of  hot  hydrochloric 
acid  upon  some  iron-bearing  material  deep  within  the  earth's  in- 
terior. This  being  volatile  at  high  temperatures  escapes  to  the  air 
as  a  gas.  Here  it  may  react  with  water  vapor,  with  the  resulting  for- 
mation of  hematite  (2FeCl3+3H20=Fe203+6HCl).  By  the  action 
of  carbonic  acid  gas  upon  volatile  oxides,  carbonates  are  formed, 
(Fe203+2C02=2FeCC>3+0).  In  other  cases,  however,  the  reactions 
are  very  complicated. 

Precipitation  from  Solution. — Nearly  all  substances  are  soluble 
to  an  appreciable  degree  in  pure  water.  An  increase  in  temperature 
usually  increases  the  quantity  of  the  substance  that  can  be  dissolved, 
as  does  also  an  increase  of  pressure.  Moreover,  the  solubility  of  a 
salt  is  increased  on  the  addition  of  another  salt  containing  no  common 
ion,  and,  conversely,  is  diminished  in  the  presence  of  another  having  a 
common  ion.  Thus,  gypsum  (CaS04*2H20)  is  sparingly  soluble  in 
water,  but  it  becomes  much  more  soluble  upon  the  addition  of  salt 
(NaCl).  On  the  other  hand,  salt  (NaCl)  is  much  less  soluble  in  water 
containing  a  little  magnesium  chloride  (MgCb)  than  it  is  in  pure  water. 

When  a  solvent  contains  a  maximum  amount  of  any  substance  that 
it  may  hold  under  a  given  set  of  conditions  the  solution  is  said  to  be 
saturated.  From  a  saturated  solution  under  ordinary  conditions 
precipitation  results:  Upon  the  evaporation  of  the  solvent;  the  lowering 
of  its  temperature  or  of  the  pressure  under  which  it  exists;  or  the  addi- 
tion to  the  solution  of  a  substance  containing  an  ion  already  in  the 
solution.  Of  course,  the  addition  of  a  substance  which  will  react  with 
the  solution  and  produce  a  compound  insoluble  in  it  will  also  cause 
precipitation. 

The  following  table  contains  the  results  of  various  experiments  on 
the  solubility  of  some  common  minerals: 

Solubility  of  Various  Compounds  in  100  Parts  Pure  Water 

(The  results  are  given  in  parts  by  weight) 

Halite  (NaCl),  at  70 35.68  Calcite  (CaC02),  in  the 

Fluorite  (CaFj),  at  15 J° 0037  c°ld 002 

Gypsum  (CaS04*2H20),  at  150       .250  Strontianite   (SrC03)  in 

Anhydrite  (CaS04),  in  the  cold       .00025  the  cold °o555 

Celestite  (SrS04),  at  140 015  Magnetite  (F3e04) 00035 


FORMATION  OF  MINERALS  19 

Percentages  of  Various  Minerals  Soluble  in  Water  at  80° 

(When  treated  30  to  32  days) 

Galena  (PbS) 1 .  79  Chalcopyrite  (CuFeSi) 1669 

Stibnite  (SbA) 5 .01  Bournonite  ((Pb -Cu)SbSa). . . 2 .075 

Pyrite  (FeSj) 2 .  99  Arsenopyrite  (FeAsS) 1.5 

Sphalerite  (ZnS) 025 

So  many  substances  that  are  usually  regarded  as  insoluble  are  known 
to  be  dissolved  under  conditions  of  high  temperature  and  pressure  that 
no  substance  is  believed  to  be  entirely  insoluble. 

Powdered  apophyllite  ((HK)2Ca(Si03)2-H20),  which  is  a  silicate 
that  is  generally  regarded  as  insoluble  in  water,  is  dissolved  sufficiently 
in  this  solvent  at  a  temperature  of  i8o°-i90°  and  under  a  pressure  of 
10-12  atmospheres  to  yield  crystals  of  the  same  substance  upon  cooling. 

Water  containing  gases  or  traces  of  salts  is  usually  a  more  efficient 
dissolving  agent  than  pure  water.  When  the  gases  are  lost,  or  the 
salts  are  decomposed  by  reactions  with  other  compounds,  precipitation 
may  ensue. 

Parts  of  Various  Minerals  Dissolved   in  10,000  Parts   of  Various 

Solutions 

Gold  loses  1.23  per  cent  of  its  weight  when  treated  with  10  per  cent  soda 
solution  at  2000. 

One  part  gypsum  (CaSO^HjO)  dissolves  in  199  parts  of  saturated  NaCl 
solution.     Only  .4  part  dissolves  in  200  parts  pure  water. 

Pyrite  (FeSa)  loses  10.6  per  cent  of  its  mass  upon  boiling  for  a  long  time 
with  a  solution  of  Na*S.  Under  the  same  circumstances  galena  loses  2.3 
per  cent. 

One  of  the  commonest  of  the  gases  found  in  water  on  the  earth's 
surface  is  carbon  dioxide.  This  is  an  active  agent  in  decomposing  sili- 
cates and  in  dissolving  carbonates,  so  that  water  in  which  it  is  dissolved 
is  usually  a  more  powerful  solvent  than  pure  water.  Its  dissolving 
power  increases  with  the  pressure,  as  in  the  case  of  pure  water,  but 
diminishes  with  increasing  temperature.  The  action  of  carbonated 
water  on  silicates  is  due  to  the  replacement  of  the  silicic  acid  by  carbonic 
acid  and  the  production  of  bicarbonates,  which  are  usually  more  soluble 
than  the  corresponding  carbonates.  The  greater  solubility  of  carbon- 
ates, like  calcite,  in  carbonated  water  is  also  due  to  the  formation  of 
bicarbonates.  For  example,  the  action  of  carbonated  water  upon  cal- 
cite (CaCOs)  is  as  follows: 

CaC03+H20+C02= CaH2(C03)2. 


20  GENERAL  CHEMICAL  MINERALOGY 

Carbonated  water  is  more  effective  as  a  solvent  under  pressure 
because  of  the  inability  of  the  CO2  to  escape  under  this  condition.  When 
pressure  is  removed  the  CO2  escapes,  or  evaporation  takes  place,  and  the 
reverse  reaction  occurs,  as: 

CaH2(C03)2  =  CaC03+H20+C02. 

The  dissolving  effect  of  carbonated  water  upon  various  carbonates 
and  other  minerals  and  the  influence  of  pressure  and  temperature  upon 
the  solution  of  a  carbonate  are  indicated  in  the  three  tables  following: 

Solubility  of  Certain  Carbonates  in  10,000  Parts  of    Carbonated 

Water 

(The  results  are  given  in  parts  by  weight) 

Calcite  (CaCO,),  at  io° 10. o        Siderite  (FeCOs)  at  180 7.2 

Dolomite  (CaMg(COa)a)  at  180.    3 . 1        Witherite  (BaCO,)  at  io° 17.0 

Magnesite  (MgCO»),  at  50.  . . .   13. 1        Strontianite  (SrCOs),  at  io°.  .    12.0 

Percentages  of  Various  Minerals    Soluble   in    Carbonated  Water 

(When  treated  7  weeks) 

Adularia  (KAlSi,Og) 328  Apatite  (Ca»(F- CI) (P04)a).. .   1.821 

Oligoclase  Apatite  (Ca»(F  •  CI)  (PO<),) ...   2.018 

(NaAlSi,08+CaAl(SiO)4). . .     .533  Olivine  ((Mg.Fe)2Si04) 2 .  in 

Hornblende  (complex  silicate)  1.536  Magnetite  (Fe304). . .    .307  to  1 .821 

Serpentine  (HiMgaSijO*) 1 .  211 

Influence    of    Temperature    and    Pressure  upon   the  Solution  of 
Magnesium  Carbonate  (MgCOi)  in  Carbonated  Water 
(The  results  are  given  in  parts  per  10,000  by  weight) 
1  atmos.  at  190 . .   2 .  579  parts        Temp.   13 .4°  under  1  atmos. .  2 .845  parts 
32  3-730  29.3  2.195 

5.6  4.620  62.0  1 .035 

75  5120  82.0  .490 

9.0  5659  100. 0  .000 

Precipitation  from  Atmospheric  Water. — Rain  is  an  active  agent 
in  dissolving  mineral  matter.  Since  it  absorbs  small  quantities  of  carbon 
dioxide,  sulphur  gases  and  other  substances  as  it  passes  through  the 
atmosphere  it  may  act  upon  many  compounds,  dissolving  some,  decom- 
posing others  and  forming  soluble  compounds  from  those  that  would 
otherwise  be  practically  insoluble.  Moreover,  it  transports  the  dissolved 
materials  from  one  portion  of  the  crust  to  some  other  portion,  where, 
under  favorable  conditions,  they  may  be  precipitated.  The  rain  water 
that  penetrates  the  earth's  crust,  dissolving  and  precipitating  in  its 


FORMATION   OF   MINERALS  21 

course  through  the  crust,  is  known  as  vadose  water.  It  is  an  important 
agent  in  ore-formation,  since  it  may  collect  mineral  matter  from  a  great 
mass  of  rocks  and  precipitate  it  in  some  favorable  place,  thus  making 
ore  bodies. 

Deposits  of  Springs. — Springs  are  the  openings  at  which  under- 
ground water  escapes  to  the  earth's  surface.  Much  of  the  water  flowing 
from  springs  is  the  meteoric  water  which  has  circulated  through  the 
crust  and  is  again  seeking  the  surface.  In  its  course  through  the  crust  it 
dissolves  certain  materials.  Where  it  reaches  the  surface  some  of  this 
material  may  be  dropped  in  consequence  of  (i)  evaporation  of  the  water, 
or  (2)  the  escape  of  carbon  dioxide,  or  (3)  the  oxidation  of  some  of  its 
constituents  through  the  action  of  the  air,  or  (4)  the  cooling  of  the  water 
in  the  case  of  warm  or  hot  springs. 

The  deposits  thus  formed  may  occur  as  thin  coatings  on  the  rocks 
over  which  the  spring  water  passes,  or  as  layers  in  the  bottom  of  the 
spring  and  the  stream  issuing  from  it.  Among  the  commonest  minerals 
thus  deposited  are  calcite  (CaCOa),  aragonite  (CaCCb),  siderite  (FeCOa) 
and  other  carbonates,  gypsum  (CaSO^zHaO),  pyrite  (FeSs),  sulphur 
(S),  and  limonite  (Fe403(OH)e).  The  carbonates  are  deposited  largely 
in  consequence  of  the  escape  of  CO2  from  the  water,  gypsum  in  conse- 
quence of  cooling,  and  limonite  and  sulphur  through  oxidation.  If  the 
water  contains  H2S,  this  reacts 
with  the  oxygen  and  a  deposit  .4  j 
of    sulphur     ensues     (compare     -'-^^-'. :  ^,-  '■■ 

When  the   precipitation  oc-      .'"^A' 
curs  in  cracks  or  fissures  in  the       "■  V'v.  V"-_. 

rocks  the  precipitated  matter  l.-T_- 
may  partially  or  completely  fill  _"-_"  V 
the  fissure,  producing  a  vein;  or,  ".  ^' 

the  precipitated  matter  may  fill         "J  "  - " 
an  irregular  cavern   forming  a  '..  -        x-„  ■ 

bonanza.     It  sometimes  covers  " 

the  walls  of  cavities  or  the  sur-  Fie.  3.— Cross-section  of  Symmetrical  Veto, 
faces  of  minerals  already  exist-  (After  Le  Nine  Foster.) 

ing,  giving  rise  to  a  druse.     In         (a)  De™™c™«i  ™*.  W  Galena, 

other  cases  precipitation   may 

occur  while  the  solution  is  dripping  from  an  overhanging  surface, 
making  a  stalactite,  or  the  precipitate  may  fill  the  tiny  crevices  between 
grains  of  sand  cementing  the  loose  mass  into  a  compact  rock, 

Minerals  produced  by  precipitation  are  often  beautifully  crystallized. 


! 


22  GENERAL  CHEMICAL  MINERALOGY 

At  other  times  they  form  groups  of  needles  yielding  globular  and  other 
imitative  shapes,  while  in  still  other  instances  they  occur  as  pulverulent 
or  amorphous  masses.  The  fillings  of  veins  are  often  arranged  sym- 
metrically, similar  materials  occurring  on  opposite  sides  of  their  central 
planes  in  bands,  as  shown  in  the  figure  (Fig.  3).  Some  important  ores 
have  been  concentrated  and  deposited  in  this  way. 

Deposits  from  Hot  Springs. — The  water  of  hot  springs  deposits  a 
greater  variety  of  minerals  than  that  of  cold  springs.  Practically  all 
minerals  that  are  soluble  in  hot  water  or  in  hot  solutions  of  salts  are 
among  them.  Among  those  of  economic  value  may  be  mentioned 
cinnabar  (HgS)  and  stibnite  (Sb2S3). 

Deposits  from  the  Ocean  and  Lakes. — The  water  of  the  ocean  and 
of  many  lakes  is  rich  in  dissolved  salts.  That  of  lakes,  however,  is  often 
saturated  or  nearly  so,  while  that  of  the  ocean  is  not  near  the  saturation 
point.  Consequently,  while  many  lakes  may  deposit  mineral  sub- 
stances, the  ocean  does  not  do  so  except  under  peculiar  conditions.  When 
a  portion  of  the  ocean  is  separated  from  the  main  body  of  water,  it  may 
evaporate  and  leave  all  of  its  mineral  matter  behind.  Lakes  may  also 
completely  evaporate  with  a  similar  result.  In  each  case  the  deposits 
form  layers  or  beds  at  the  bottom  of  the  basin  in  which  the  water  was 
collected. 

In  other  instances  the  water  brought  to  the  ocean  or  a  lake  may 
contain  substances  which  will  react  with  some  of  the  materials  already 
present  and  produce  an  insoluble  compound  which  will  be  precipi- 
tated. 

Of  course,  the  nature  of  the  beds  thus  formed  will  depend  upon  the 
character  and  proportions  of  the  substances  that  were  in  the  water. 
The  ocean  will  yield  practically  the  same  kinds  of  compounds  all  over 
the  world  and  the  beds  deposited  by  the  evaporation  of  ocean  water 
will  be  formed  in  nearly  the  same  succession  everywhere.  In  the  case 
of  enclosed  bodies  of  water — like  lakes  or  seas — in  which  the  composi- 
tion of  the  water  may  differ,  the  deposits  formed  may  also  differ. 

Many  of  the  deposits  formed  in  bodies  of  water  are  of  great  eco- 
nomic importance  and,  consequently,  are  extensively  worked.  Prob- 
ably the  most  important  are  the  beds  of  salt  (NaCl)  and  of  gypsum 
(CaSO^EfeO),  although  borax  (Na2B407  •  10H2O)  was  formerly 
obtained  in  large  quantity  from  the  deposits  of  some  of  the  lakes  in 
the  desert  portions  of  the  United  States. 

In  the  following  table  are  given  the  results  of  analyses  of  water  of 
the  ocean  and  of  Great  Salt  Lake,  in  Utah,  calculated  on  the  assump- 
tion that  the  elements  are  combined  in  the  manner  indicated  in  the 


FORMATION  OF  MINERALS 


23 


column  on  the  left.    The  results  of  the  analyses  of  the  waters  of  a  few 
noted  lakes  are  given  in  the  succeeding  table. 

Composition  of  Salts  Contained  in  Water  of  the  Ocean  and  Great 

Salt  Lake 

(Parts  in  iooo  of  Water) 

I  II  HI 

NaCl 27.3726  8.1163  118.628 

KC1 5921  .1339 

MgClj 33625  .6115  14.908 

CaS04 1 .3229  .9004  .858 

MgS04 2.2437  3o85S 

Na2S04 9-321 

K2S04 5363 

RbCla .0190  .0034 

MgBr3 0547  .0081  tr 

Ca«(P04)i 0156  .0021 

CaCOi 0434  .0780 

FeCOi 0019  .0011 

SiOt 0149  .0024 


35-°433 


12.9427 


149.078 


I.  Water  of  N.  Atlantic  off  Norwegian  Coast.    Analyst,  C.  Schmidt. 
II.  Average  of  Five  Analyses,  Caspian  Sea  at  depths  of  from  1  m.  to  640  m. 

Analyst,  C.  Schmidt. 
III.  Great  Salt  Lake,  Utah.    Analyst,  O.  D.  Allen. 


Percentage  Composition  of  the  Residues  of  a  Few  Lake  Waters 


Dead  Sea 

Lake  Beisk,  Siberia 

Goodenough  Lake,  B.  C. 
Borax  Lake,  Cal 


CI 

Br 
1.45 

SO4 

CO3 

Na 

K 

3.24 

Ca 

Mg 

1 

SiO,. 

etc. 

64.49 

.45 

15.75 

4.09 

10.53 

tr 

22.70 

tr 

42.32 

.61 

31.32  1.01 

.07 

1.86 

.02 

7.64 

7.08 

41.41 

36.17 

6.65 

.02 

.04 

.99 

32.27 

.04 

.13 

22.47 

38.10 

1.52 

.03 

.35 

.02 

Total  Solida 

(per  1000 

of  Water). 


220.3 
104.7 
103.47 
76.56 


Deposits  from  Magmatic  Water. — Equally  important  in  depositing 
mineral  matter  is  the  water  that  escapes  from  cooling  lavas  and  other 
molten  magmas — designated  as  juvenile  water.  All  molten  magmas 
existing  under  pressure,  i.e.,  at  some  distance  beneath  the  crust,  contain 
the  components  of  water,  which  escape  as  the  magma  cools  or  when  the 
pressure  diminishes,  whether  the  diminution  of  the  pressure  is  due  to 


24  GENERAL  CHEMICAL  MINERALOGY 

the  escape  of  the  lava  to  the  surface  or  to  the  cracking  of  the  crust. 
In  its  passage  to  the  surface  the  hot  water  carrying  dissolved  salts  pene- 
trates all  the  cracks  and  cavities  in  the  rocks  through  which  it  passes 
in  its  ascent  and  deposits  its  burden  of  material,  forming  veins  and  other 
types  of  deposits.  Or,  its  components  may  decompose  the  materials 
with  which  it  comes  in  contact,  replacing  them  wholly  or  in  part  by  the 
substances  which  it  is  carrying  or  by  the  products  of  decomposition. 


FlO.  4. — Croas-section  of  Vein  in  Green  Porphyry.  The  vein  filling  is  chalcedony. 
The  white  splotches  are  feldspar  crystals.  The  [airly  uniform  character  of  the 
rock  where  not  affected  by  the  vein  is  seen  on  the  right  side  of  the  picture.  The 
rude  banding  parallel  to  the  vein  is  due  to  changes  that  have  proceeded  out- 
ward from  tbe  vein-mass  into  the  rock. 

Since  in  many  cases  magmatic  water  contains  corrosive  gases,  such  as 
fluorine,  its  action  on  the  rocks  which  it  traverses  is  profound.  A  tiny 
crack  in  the  rocks  may  be  gradually  widened  and  the  material  on  both 
sides  of  it  be  replaced  by  new  material,  thus  producing  a  vein  which 
is  sometimes  difficult  to  distinguish  from  a  vein  made  in  other  ways 
(Fig.  4).  This  process  is  known  as  metasomatism,  which  is  one  kind  of 
metamorpkism.  It  is  an  important  means  of  producing  pseudomorphs 
and  bodies  of  mineral  matter  sufficiently  rich  in  metallic  contents  to 
constitute  ore-bodies. 


FORMATION  OF  MINERALS  25 

Solidification  from  Molten  Magmas. — A  molten  magma,  such  as  a 
liquid  lava,  is  probably  a  solution  of  various  substances — mainly  sili- 
cates— in  one  another,  or  in  a  hot  solvent.  Upon  cooling  or  upon  change 
of  conditions,  such  as  may  arise  from  loss  of  gas  or  water  or  from  reduc- 
tion of  pressure,  this  hot  solution  gradually  deposits  some  of  its  con- 
stituents as  definite  chemical  compounds.  Upon  further  cooling  other 
compounds  solidify  and  so  on,  until  finally,  if  the  rate  of  cooling  has  been 
slow,  the  entire  mass  may  separate  as  an  aggregate  of  minerals — such 
as  constitute  many  of  the  rocks,  as  granite  for  instance,  and  many  of  the 
lavas.  If  the  cooling  has  been  rapid,  some  of  the  material  may  separate 
as  definite  minerals  while  the  remainder  solidifies  as  a  homogeneous 
glass,  as  in  the  case  of  most  lavas.  Sometimes  the  minerals  thus  formed 
are  bounded  by  crystal  planes,  but  usually  their  growth  has  been  so 
interfered  with  that  it  is  only  by  their  optical  properties  that  they  can 
be  recognized  as  crystalline  substances.  The  nature  of  the  minerals 
that  separate  depends  upon  a  great  variety  of  conditions,  the  most 
important  of  which  is  the  chemical  composition  of  the  magma. 

In  some  cases  the  minerals  separating  from  a  magma  tend  to  segre- 
gate in  some  limited  portion  of  its  mass  and  thus  produce  an  accumula- 
tion that  may  be  of  economic  value,  i.e.,  the  magma  differentiates. 
Magnetite  (FeaO^,  ilmenite  ((Fe-Ti)203),  pyrite  (FeS2)  and  a  few  other 
minerals  are  sometimes  segregated  in  this  way  in  very  large  masses. 

Metamorphic  Minerals. — Many  minerals  are  characteristic  of  rocks 
that  are  in  contact  with  others  that  were  once  molten.  They  were 
formed  by  the  gases  and  hot  waters  given  off  from  the  magmas  before  they 
cooled.  The  hot  solutions  with  their  charges  of  gas  and  salts  penetrated 
the  pores  of  the  surrounding  rock  and  deposited  in  them  some  of  their 
material.  They  reacted  with  some  of  the  rock's  components,  producing 
new  compounds,  and  extracted  others,  leaving  pores  into  which  new 
supplies  of  gas  and  water  might  enter.  In  some  cases  the  entire  body 
of  the  surrounding  rock  has  been  replaced  by  new  material  for  some 
distance  from  the  contact.  Beyond  this  belt  of  most  profound  meta- 
morphism  are  other  belts  in  which  the  rock  is  less  altered,  until  finally  in 
the  outer  belt  is  the  unchanged  original  rock.  Into  the  outer  contact 
belt  perhaps  only  gas  penetrated  and  the  changes  here  may  be  entirely 
pneumatolytic.  Near  the  contact  the  changes  may  be  metasomatic. 
Minerals  formed  by  these  processes  near  the  contact  of  igneous  masses 
are  frequently  referred  to  collectively  as  contact  minerals. 

In  other  cases  new  minerals  may  be  produced  in  rocks  in  consequence 
of  crushing  attended  by  heat.  Hot  water  under  high  pressure 
greatly  facilitates  chemical  changes.    A  part  of  the  materials  of  the 


*  .# 


26  GENERAL  CHEMICAL  MINERALOGY 

crushed  rock  dissolves,  reactions  are  set  up  and  new  compounds  may 
be  formed.  The  new  minerals  produced  are  more  stable  than  the 
original  ones  and  have  in  general  a  greater  density  and  consequently 
a  smaller  volume.  The  type  of  metamorphism  that  produces  these 
effects  is  known  as  dynamic  metamorpkism. 

Organic  Secretions. — The  transfer  of  mineral  substances  from  a 
state  of  solution  to  the  solid  condition  is  often  produced  through  the  aid 
of  organisms.  Mollusca,  like  the  oyster,  clam,  etc.,  crustaceans,  like 
the  lobster  or  crab,  the  microscopic  animals  and  plants  known  as  pro- 


Fig.  5. — Diorite  Dike  Cutting  Granite  Gneiss,     Pelican  Tunnel,  Georgetown,  Colo. 

(After  Spun  and  Carry.) 

tozoans  and  algae  and  many  other  animals  and  vegetables  abstract 
mineral  matter  from  the  water  in  which  they  live  and  build  up  for  them- 
selves hard  parts.  These  hard  parts,  usually  in  the  form  of  external 
shells,  are  composed  of  calcium  carbonate  (CaCOa),  either  as  calcite  or 
aragonite,  of  silica  (SiOz)  or  of  calcium  phosphate  Ca3(PO.j)2.  Although 
not  commonly  regarded  as  minerals  these  substances  are  identical 
with  corresponding  substances  produced  by  inorganic  agencies.1 

Paragenesis. — -It  is  evident  that  minerals  produced  in  the  same 

1  Plants  and  animals  upon  decaying  yield  organic  acids  which  may  attack  minerals 

already  existing  and  thus  give  rise  to  solutions  which  may  deposit  pyrite  (FeSj), 

limonite  (a  hydrated  iron  oxide)  or  some  other  metallic  compound.     This  process, 

however,  is  property  simply  a  phase  of  deposition  from  solutions. 


FORMATION  OF  MINERALS  27 

way  will  generally  be  found  together.     A  certain  association  of  minerals 
will    thus   characterize   deposits   from   magmas,   another   association 


Fig.  6.— Vein  in  Griffith  Mine,  Georgetown,  Colo.,  Showing  Two  Periods  of  Vein 

Deposition.     (After  Sfiurr  and  Carry.) 

(T\  -  wall  rock.  b  =-  sphalerite.  c  -  chalcopynto. 

q  -  comb  quarts.  P  —  pyrite.  g  —  galena. 

Balance  of  vein-filling  is  a  mixture  of  manganese-iron  carbonates. 


Fig.  7.    Vein  Forming  Original  Ore-Body,  Butte,  Mont.    (After  W.  H.  Weed.) 

(0  P«ult  breccia;   (2)  ore;   (3)  altered  granite;    (4)  firsl-dasa  ore;   (5)  crushed  quarti  and 
wrntte;   (6)  fault  clay;    (7)  solid  pyrite  and  bornite;    (S)  crushed  quartz  and  pyrite;   (9)  solid 


those  precipitated  from  water,   another  those  produced  by  contact 
action,  etc.    This  association  of  minerals  of  a  similar  origin  is  known 


28  GENERAL  CHEMICAL  MINERALOGY 

as  their  paragenesis.     From  a  study  of  their  relations  to  one  another  the 
order  of  their  deposition  may  usually  be  determined. 

Occurrence. — The  manner  of  occurrence  of  mineral  substance  is 
extremely  varied,  as  may  be  judged  from  the  consideration  of  the  vari- 
ous ways  in  which  they  are  formed.    Deposits  laid  down  in  water  occur 
in  beds  or  in   the  cement  uniting  grains  of  sand,  etc.,  such  as  the  beds 
of  salt  (NaCl)  or  gypsum  (CaS04  ■  2H2Q)  found  in  many  regions.    Those 
produced  by  the  cooling  of  magmas  may  form  great  masses  of  rock 
such  as  granite,  which  when  it  occurs  as  the  filling  of  cracks  in  other 
rocks  is  said   to  have  the  form  of  a  dike  (Fig.  5)-     Deposits  made  by 
water,   whether  meteoric   or  mag- 
matic  may  give  rise  to  veins,  which 
may  be  straight-walled  or  branch- 
ing, like  the  veins  of  quartz  (SKfe) 
that  are  so  frequently  seen  cutting 
various  siliceous  rocks.    When  the 
veins  are  filled  by  meteoric  water 
they  often  have  a  comb-structure — 
the  filling  consisting  of  several  sub- 
stances arranged  in  definite  layers 
following  the  vein  walls  (see  p.  21). 
If  the  composition  of  the  depositing 
solution,  whether  meteoric  or  mag- 
matic,  has  remained  constant  for  a 
long  time   the  vein  may  be  filled 
with  a  single  substance.     If  its  com- 
position  changed   during  the  time 
the  filling  was  in  progress  the  layers 
are  of  different  kinds.    Further,  it 
Fig.  8.— Druse  of  Smithsonite  (ZnCft)    deposUioncontmueduninterruptedly 
on  Massive  Smithsonite.  the  layers  may  match  on  opposite 

sides  of  the  vein  and  the  succession 
may  be  the  same  from  walls  to  center.  If,  however,  after  the  partial 
or  complete  filling  of  the  crack  it  was  reopened  and  the  new  crack  was 
filled,  the  new  vein  when  filled  would  be  unsymmetrical  if  the  new  crack 
occurred  to  one  side  of  the  center  of  the  original  vein  (Fig.  6).  Repeated 
reopening  may  give  rise  to  a  vein  that  is  so  lacking  in  symmetry  that 
it  is  difficult  to  trace  the  succession  of  events  by  which  it  was  produced 
(Fig-  7)-  Veins  filled  by  inagmatic  water  are  frequently  more  homo- 
geneous. 

Druses  (Fig.  8)  arise  when  deposits  simply  coat  the  walls  of  fissures. 


FORMATION  OF  MINERALS  29 

In  many  cases  they  may  be  regarded  as  veins,  the  development  of  which 
has  been  arrested  and  never  completed.  When  the  deposits  coat  the 
walls  of  hollows  within  rocks  they  are  known  as  geodes  (Fig.  o).  Geodes 
are  common  in  limestones  and  other  easily  soluble  rocks  in  which 
cavities  may  be  dissolved. 

Gases  and  water  under  great  pressure  may  penetrate  the  micro- 
scopic pores  existing  in  all  rocks  and  there  deposit  material  which  may 
fill  the  pores  and  cement  the  rocks.  If  the  deposited  material  is  metallic 
the  rocks  may  be  transformed  into  masses  sufficiently  rich  in  metallic 
matter  to  become  ore-bodies.  A  body  of  this  kind  is  known  as  an 
impregnation.  It  is  well  represented  by  some  of  the  low  grade  gold 
ores,  such  as  those  in  the  Black  Hills. 

When  rocks  are  decomposed  by  the  weather  they  are  broken  up. 


Fig.  q.— Geodes  Containing  Calcite  (CaCOi)  Crystals. 

The  rains  wash  the  disintegrated  substance  into  streams.  In  its  course 
downward  to  lakes  or  the  ocean,  the  heavier  fragments,  such  as  metallic 
particles,  may  settle  while  the  lighter  portions  are  carried  along. 
Thus  the  heavy  parts  may  accumulate  in  the  stream  bottoms.  These 
materials,  consisting  of  gold,  magnetite,  garnet,  pyrite  and  other  min- 
erals of  high  specific  gravity,  form  a  loose  deposit  in  the  stream  bed 
which  is  known  as  a  placer.  Gold  is  often  found  in  placer  deposits. 
The  lighter  portions  may  be  carried  to  the  lake  or  sea  into  which  the 
streams  enter  and  may  accumulate  as  sand  on  beaches  and  on  the 
bottom  near  the  shores  as  gravel,  sand,  silt,  etc.  Most  sand  consists 
principally  of  quartz,  but  many  sands  contain  also  grains  of  feldspar 
and  other  silicates,  and  sometimes  other  compounds. 


30  GENERAL  CHEMICAL  MINERALOGY 

Alteration  of  Minerals. — Minerals,  like  living  things,  are  constantly 
subject  to  change.  Circulating  waters  may  dissolve  them  in  part, 
or  completely,  and  transport  their  material  to  a  distant  place,  there 
depositing  it  either  in  the  form  it  originally  possessed  or  in  some  new 
form.  On  the  other  hand,  the  mineral  substance  may  be  decomposed 
into  several  compounds  some  of  which  may  be  carried  off,  while  others 
are  left  behind.  Again,  the  material  remaining  behind  may  com- 
bine with  other  matter  held  in  the  water  causing  the  decomposition, 
and  may  form  with  it  a  new  mineral  or  a  number  of  different  minerals 
occupying  the  place  of  the  original  one.    This  is  in  part  metasomatism. 

The  atmosphere  may  also  act  as  a  decomposer  of  minerals.  Through 
the  agency  of  its  oxygen  it  may  cause  their  oxidation,  or  it  may  cause 
them  to  break  up  into  several  oxidized  compounds.  Through  the  agency 
of  its  moisture,  it  may  dissolve  some  of  these  secondary  substances  or 
it  may  form  with  them  hydrated  compounds.  The  substances  thus 
formed  may  be  dissolved  in  water  and  carried  off,  or  they  may  remain 
to  mark  the  place  of  the  mineral  from  which  they  were  derived. 

Water,  containing  traces  of  salts,  or  gases  in  solution  are  exceedingly 
active  agents  in  effecting  changes  in  minerals.  Many  examples  of  the 
alteration  of  practically  insoluble  minerals  under  the  influence  of  dilute 
solutions  are  known.  Calcite  (CaCOa),  for  instance,  when  acted  upon 
by  a  solution  of  magnesium  chloride  (MgCb)  takes  up  magnesium  and 
loses  some  ©f  its  calcium.  Monticellite  (CaMgSi04)  when  acted  upon 
by  solutions  of  alkaline  carbonates  breaks  up  into  a  magnesium  silicate 
and  calcium  carbonate.  Dilute  solutions  of  various  salts  are  constantly 
circulating  through  the  earth's  crust  and  are  there  effecting  trans- 
formations in  the  minerals  with  which  they  come  in  contact.  On,  or 
near,  the  surface  the  transformations  are  taking  place  more  rapidly 
than  elsewhere  because  here  the  solutions  are  aided  in  their  decompos- 
ing action  by  the  gases  of  the  atmosphere. 

The  effect  of  the  air  in  causing  alteration  is  seen  in  the  green  coat- 
ing of  malachite  ((CuOH^COs)  that  covers  surfaces  of  copper  or  of 
copper  compounds  exposed  to  its  action.  In  this  particular  case  the 
coating  is  due  to  the  action  of  the  carbon  dioxide  and  the  moisture  of 
the  atmosphere.  Other  substances  in  contact  with  the  air  are  coated 
with  their  own  oxides,  sulphides,  etc. 

Pseudomorphs. — When  the  alteration  of  a  mineral  has  proceeded 
in  such  a  manner  that  the  new  products  formed  have  replaced  it  particle 
by  particle  a  pseudomorph  results.  Sometimes  the  newly  formed  sub- 
stance crystallizes  as  a  single  homogeneous  grain  filling  the  entire 
space  occupied  by  the  original  substance.    Usually,  however,  the  alter- 


FORMATION  OF  MINERALS  31 

ation  begins  along  the  surfaces  of  cracks  or  fissures  in  the  body  under- 
going alteration,  or  upon  its  exterior,  thus  producing  the  new  material 
at  several  places  contemporaneously  (Fig.  10).  When'  the  replace- 
ment takes  place  in  this  manner  the  resulting  mass  is  a  network  of 
fibers  of  the  new  substance  or  an  aggregate  of  grains  with  the  outward 
form  of  the  replaced  mineral. 

With  respect  to  their  method  of  formation  chemical  pseudomorphs 
may    be    classified    as    alteration 
pseudomorphs     and     replacement 
pseudomorphs. 

Alteration  Pseudomorphs.  — 
Pseudomorphs  of  this  class  may 
be  denned  as  those  which  retain 
some  or  all  of  the  constituents  of 
the  original  minerals  from  which 
they  were  derived. 

Paramorphs.  —  Pseudomorphs 
composed  of  the  material  of  the 

pseudomorphed  substance  with-  Flc.  IO_ Alteration  of  Olivine  into  Ser- 
out  addition  or  subtraction  of  pentine.  The  alteration  is  proceeding 
any  component  are  known  as  from  the  surface  of  the  crystal  and 
paramorphs.  from  surfaces  of  cracts  that  traverse 

n  ,.         .  .,,  ,  it.      The    black    specks   and  streaks 

Faramorphism  is  possible  only  ,      __.-.,        .  .    -     .. 

r  r  J  represent  magnetite  formed  during  the 

in  the  case  of  dimorphous  bodies.        ptoeess.    (After  Tschtrmak.) 
It  results  from  the  rearrangement 

into  new  bodies  of  the  particles  of  which  the  original  body  was  com- 
posed. 

Illustrations:  CalcUe  (hexagonal  CaCOa)  after  aragonite  (ortho- 
rhombic  CaCOa);   orthorhombic  sulphur  after  the  monoclinic  variety. 

Partial  Pseudomorphs. — The  great  majority  of  pseudomorphs 
retain  a  portion,  but  not  all,  of  the  material  of  the  original  mineral. 
They  may  be  formed  by  the  addition  of  material  to  the  original  body; 
by  the  loss  of  material  from  it;  or  by  the  replacement  of  a  portion  of 
its  material  by  new  material. 

Pseudomorphs  formed  by  the  addition  of  substance  to  that  already 
existing  are  rare.  The  substances  most  frequently  added  in  the  pro- 
duction of  such  pseudomorphs  are  oxygen,  sulphur,  the  hydroxyl 
group  (OH)  and  the  carbonic  acid  group  (CO3  and  CO2). 

Illustrations:  Malachite  ((CuOH)2C03)  after  copper,  and  argentile 
(AgzS)  after  silver. 

Pseudomorphs  resulting  from  the  loss  of  material  are  not  common. 


32  GENERAL  CHEMICAL  MINERALOGY 

They  are  caused  by  the  abstraction  of  one  or  more  of  the  constituents 
of  a  compound. 

Illustration:  Native  copper  after  cuprite  (CU2O). 

The  greater  number  of  partial  pseudomorphs  are  formed  by  the  sub- 
stitution of  some  of  the  components  of  the  original  mineral  by  a  new 
material. 

Illustrations:  Limonite  (Fe403(OH)e)  pseudomorphs  after  siderite 
(FeC03)  may  be  formed  by  the  following  reaction: 

4FeC03+  2O+3H2CM  4C02+Fe403(OH)6. 
Cerussite  (PbC03)  may  be  formed  from  galena  (PbS),  thus: 
PbS+40+Na2C03  =  PbC03+Na2S04. 

Replacement  Pseudomorphs. — Often  the  entire  substance  of  a 
mineral  is  replaced  by  new  material,  so  that  no  trace  of  its  original 
matter  remains.  In  this  case  the  nature  of  the  pseudomorphed  min- 
eral can  be  discovered  only  from  the  form  of  the  pseudomorph. 

Illustrations:  Quartz  (S1O2)  after  calcite  (CaC03)  and  gypsum 
(CaS04-2H20)  after  halite  (NaCl). 

Mechanical  Pseudomorphs. — The  processes  described  above  as 
originating  pseudomorphs  are  chemical,  and  the  resulting  pseudomorphs 
are  sometimes  designated  chemical  pseudomorphs.  There  is  another 
class  of  pseudomorphs,  however,  in  which  the  substance  of  a  crystal 
has  not  been  replaced  gradually  by  the  pseudomorphing  substance. 
In  this  class  the  pseudomorphing  substance  simply  fills  a  mold  left  by 
the  solution  of  some  preexisting  crystal.  Thus,  if  a  sulphur  crystal 
should  become  encrusted  with  a  coating  of  barite  (BaS04)  and  the 
temperature  should  rise  until  the  sulphur  melts  and  escapes,  there 
would  be  left  a  mold  of  itself  constructed  of  barite.  If,  now,  a  solution 
of  calcium  carbonate  should  penetrate  the  cavity  and  fill  it  with  a  deposit 
of  calcite  (CaC03),  the  mass  of  calcite  would  have  the  shape  of  a  crystal 
of  sulphur.  Pseudomorphs  of  this  kind  are  known  as  mechanical 
pseudomorphs. 

Weathering. — The  term  weathering  is  applied  to  the  sum  of  all  the 
changes  produced  in  minerals  by  the  action  of  the  atmosphere  upon 
them.  Although  nearly  all  minerals  show  some  traces  of  weathering, 
these  traces  may  often  be  detected  only  by  the  slight  differences  in  color 
exhibited  by  surfaces  that  have  been  exposed  for  a  long  time  to  the 
action  of  the  air  when  compared  with  fresh  surfaces  produced  by  frac- 
ture or  cleavage. 


FORMATION  OF  MINERALS  33 

The  weathering  of  minerals  is  often  of  great  economic  importance. 
Veins  of  sulphides  and  a  few  other  compounds  may  be  oxidized  where 
they  outcrop  on  the  surface.  Some  of  the  decomposition  products  thus 
formed  may  be  soluble  and  others  insoluble.  The  insoluble  products 
may  remain  near  the  surface  while  the  soluble  ones  are  carried  down- 
ward by  ground  water  along  the  course  of  the  vein.  Here  a  reaction 
may  ensue  between  the  soluble  salts  and  the  undecomposed  portion  of 
the  vein  with  the  result  that  metallic  compounds  may  be  precipitated, 
thus  enriching  the  original  vein  matter  and  causing  it  to  be  changed 
from  a  comparatively  lean  ore  to  one  of  great  richness. 

Pyrite  veins  on  the  surface  are  often  marked  by  accumulations  of 
limonite  derived  by  the  oxidation  of  the  sulphide.  With  this  may  be 
mixed  insoluble  carbonates,  silicates  and  other  salts  of  valuable  metals 
present  in  the  original  sulphide.  Weathering  may  extend  downward 
along  the  veins  for  a  short  distance,  replacing  their  upper  portions  with 
the  oxidized  decomposition  products.  This  portion  of  a  vein  is  often 
spoken  of  as  the  oxidized  zone,  and  this  is  sometimes  the  richest  portion 
of  the  vein.  It  may  be  rich  because  less  valuable  substances  have 
formed  soluble  salts  and  have  been  drained  away. 

Below  the  oxidized  zone  may  be  another  zone  less  rich  in  valuable 
compounds  than  the  oxidized  zone,  but  much  richer  than  the  material 
below  it.  The  soluble  decomposition  products  of  the  upper  portion  of 
the  vein  may  percolate  downward,  and  react  with  the  unchanged  vein 
matter,  precipitating  valuable  metallic  salts.  Although  the  original 
vein  matter  may  contain  an  inconsiderable  quantity  of  the  valuable 
material,  the  precipitation  in  it  of  additional  stores  of  material  of  the 
same  kind  may  raise  the  percentage  of  this  constituent  to  a  point  where 
it  is  profitable  to  mine  it.  This  belt  of  enriched  ore  is  known  as  the 
zone  of  secondary  enrichment. 

The  oxidized  zone  extends  downward  from  the  surface  to  a  depth  at 
which  the  atmosphere  and  meteoric  water  become  exhausted  of  their 
oxygen — a  depth  which  varies  with  local  conditions.  The  zone  of 
secondary  enrichment  extends  from  the  bottom  of  the  oxidized  zone 
to  a  short  distance  below  the  level  of  the  ground  water,  beyond  which 
solutions  will  diffuse  and  thus  be  carried  away  from  the  vein.  Below 
the  zone  of  enrichment  the  original  vein-filling  may  reach  downward 
indefinite  distances. 

Since  many  veins  exhibit  the  features  described,  it  follows  that  the 
ore  of  many  mines  must  grow  poorer  with  depth,  and  that  in  many 
instances  the  richest  ore  is  near  the  surface. 

Some  of  the  changes  involved  in  weathering  and  secondary  enrich- 


34  GENERAL  CHEMICAL  MINERALOGY 

ment  of  sulphide  veins  in  limestone  are  indicated  by  the  following  reac- 
tions in  the  case  of  a  vein  containing  pyrite  (FeS2),  sphalerite  (ZnS), 
and  galena  (PbS). 

(i)  The  first  change  produced  at  the  surface  may  be  the  oxidation 
of  the  sulphides  to  sulphates. 

(a)  ZnS+40=ZnS04; 

(b)  PbS+40=PbS04  (anglesite); 

(c)  FeS2+70+H20=H2S04+FeS04. 

(2)  These  may  react  with  the  limestone  as  follows: 

(smithsonite)  (gypsum) 

(a)  ZnS04+CaC03+2H20=ZnCOs    +     CaS04-2H20; 

(cerussite)  (gypsum) 

(b)  PbS04+CaC03+2H20=PbC03     +     CaS04-2H20. 

(3)  Some  of  the  sulphates  and  carbonates  carried  down  into  the  un- 
altered sulphides  may  react  with  these,  yielding: 

(galena) 

(a)  PbS04+FeS2+02=PbS+FeS04+S02; 

(galena)      (siderite) 

(b)  PbC03+FeS2+02=PbS    +    FeC03    +    S02; 

(galena) 

(c)  PbS04+ZnS  =  PbS+ZnS04; 

(galena)     (smithsonite) 

(d)  PbC03+ZnS  =  PbS    +    ZnCOs. 

The  PbS  replacing  the  ZnS  and  deposited  in  the  cracks  in  the  original 
mixture  of  PbS,  ZnS  and  FeS2  increases  the  percentage  of  this  compound 
in  the  vein  and  thus  enriches  it. 

There  is  also  an  increase  in  the  percentage  of  ZnS  brought  about  by 
the  reactions  between  the  zinc  salts  (1a  and  2a),  and  the  pyrite,  analogous 
to  those  between  the  lead  salts  and  pyrite  ($a  and  36).    Thus: 

(sphalerite) 
ZnS04+FeS2+02     =     ZnS    +     FeS04+S02, 

(sphalerite) 
ZnC03+FeS2+02     =     ZnS     +     FeC03+S02. 


FORMATION  OF  MINERALS  35 

The  zinc  salts  produced  in  reactions  $c  and  3<f  if  carried  downward  will 
also  have  the  opportunity  to  react  with  the  pyrite  in  the  same  way. 

If  the  ZnS  is  deposited  in  fissures  in  the  vein  matter  this  will  tend  to 
enrich  it  with  zinc. 

The  oxidized  zone  contains  (smithsonite)  ZnCCfo,  (anglesite)  PbSO^, 
(cerussite)  PbCOa  and  (limonite)  Fe2(OH)2.  The  ZnS04,  formed  also 
in  the  oxidized  zone,  is  so  readily  soluble  in  water  that  it  is  leached  from 
the  other  oxidized  compounds  and  is  carried  downward. 


4 


PART  II 
DESCRIPTIVE  MINERALOGY 


CHAPTER  III 

INTRODUCTION— THE  ELEMENTS 

Of  the  1,000  or  more  distinct  minerals  recognized  by  mineralogists 
only  a  few  (some  250)  are  common.  A  few  are  important  because  they 
constitute  ores,  others  because  they  are  components  of  rock  masses, 
and  others  simply  because  of  their  great  abundance.  Only  a  few  miner- 
alogists profess  acquaintance  with  more  than  500  or  600  minerals.  The 
majority  are  familiar  with  but  300  or  400,  relying  for  the  identification  of 
the  remainder  upon  the  descriptions  of  them  recorded  in  mineralogical 
treatises. 

Only  the  minerals  commonly  met  with  and  those  of  economic  or  of 
special  scientific  importance  are  described  in  this  book.  They  should 
be  studied  with  specimens  before  one,  in  order  that  the  relation  between 
the  descriptions  and  the  objects  studied  may  be  forcibly  realized.  Min- 
eralogy cannot  be  studied  successfully  from  books  alone.  It  is  primarily 
a  study  of  objects  and  consequently  the  objects  should  be  at  hand  for 
inspection.1 

Mineral  Names. — The  names  of  the  great  majority  of  minerals  end 
in  the  termination  "ite."  This  is  derived  from  the  ancient  Greek  suffix 
"itis"  which  was  always  appended  to  the  names  of  rocks  to  signify  that 
they  are  rocks.  The  first  portion  of  the  name,  to  which  the  suffix  is 
added,  either  describes  some  quality  or  constituent  possessed  by  the 
mineral,  refers  to  some  common  use  to  which  it  has  been  put,  indicates 
the  locality  from  which  it  was  first  obtained,  or  is  the  name  of  some 
person  intended  to  be  complimented  by  the  mineralogist  who  first 
described  the  mineral  bearing  it. 

1  Collections  of  the  common  minerals  in  specimens  large  enough  for  convenient 
study  may  be  secured  at  small  cost  from  any  one  of  the  mineral  dealers  whose 
addresses  may  be  found  in  any  mineralogical  journal. 

36 


INTRODUCTION— THE  ELEMENTS  37 

The  following  examples  taken  from  Dana  illustrate  some  of  these 
principles.  The  mineral  hematite  (Fe203)  is  so  named  because  of  the  red 
color  of  its  powder,  chlorite  (a  complicated  silicate),  because  of  its  green 
color,  siderite  (FeCOa),  from  the  Greek  word  for  iron,  because  it  con- 
tains this  metal,  magnetite  (Fe304)  after  Magnesia  in  Asia,  goethite 
(FeO(OH))  after  the  poet  Goethe. 

The  names  of  a  few  minerals  end  in  "ine,"  "ane,"  "ase,"  "ote,"  etc., 
but  the  present  tendency  is  to  have  them  all  end  in  "ite."  Occasionally, 
the  same  mineral  may  have  two  names.  This  may  be  due  to  the  fact 
that  it  was  discovered  by  two  mineralogists  working  at  the  same  time 
in  different  places,  or  it  may  be  due  to  the  fact  that  the  mineralogists  of 
different  countries  prefer  to  follow  different  precedents  set  by  the  old 
mineralogists  of  their  respective  nationalities.  For  example,  the  min- 
eral (Mg-  Fe)2Si04  is  called  olivine  by  the  Germans  and  by  most  English- 
speaking  mineralogists,  and  peridot  by  the  French.  The  Germans  follow 
the  German  mineralogist  Werner,  who  first  used  the  name  olivine  in 
1789,  while  the  French  follow  the  French  teacher  Hatiy,  who  proposed 
the  name  peridot  in  1801. 

ELEMENTS 

The  elements  that  occur  in  nature  are  few  in  number,  and  these, 
with  rare  exceptions,  do  not  occur  in  great  abundance.  They  may  be 
separated  into  the  following  groups:  the  carbon  group,  the  sulphur 
group,  the  arsenic  group,  the  silver  group,  and  the  platinum-iron 
group.  Some  of  these  comprise  only  a  single  mineral,  while  others 
comprise  six  or  seven.    Only  a  portion  of  these  are  described. 

THE  NON-METALS   AND    METALLOIDS 

CARBON  GROUP 
The  carbon  group  embraces  several  minerals  of  which  one  is  dia- 
mond, another  is  an  amorphous  black  substance  known  as  schungite, 
and  the  other  two  are  apparently  but  different  forms  of  graphite. 
The  element  may  thereupon  be  regarded  as  trimorphous.  Diamond 
and  graphite  are  both  important. 

Isometric  (hextetrahedral)  Hexagonal  (ditrigonal  scalenohedral) 

Diamond  Graphite 

Diamond  (C) 

The  diamond  is  usually  found  in  distinct  crystals  or  in  irregular 
masses,  varying  in  size  from  a  pin's  head  to  a  robin's  egg.  In  some 
cases  large  individual  pieces  are  found  but  they  are  exceedingly  rare. 


38 


DESCRIPTIVE   MINERALOGY 


Fig.  ii.— Etch  Figures  on 
Cubic  Face  of  Diamond 
Crystal.     (Afttr  Ticker- 


The  largest  ever  found,  known  as  the    Cullinan  diamond  (Fig.  16), 

weighed  3,024!  carats  or  621  grams,  or  1.37  lb., 

and  measured  112x64x51  mm-    It  was  cut 

into  nine  fine  gems  and  a  number  of  smaller 

I       ones  (Fig.  17). 

I  In  composition  the  diamond  is  pure  car- 

I  bon,  but  it  is  a  form  of  carbon  that  is  not 
ignited  and  burned  at  low  temperatures.  At 
high  temperatures,  however,  especially  when 
in  the  presence  of  oxygen,  it  burns  freely 
with  the  production  of  COa,  and,  in  the  case 
of  opaque  varieties,  a  little  ash. 

Its  crystallization  is  isometric  (hextetra- 
hedral  class),  and  the  forms  on  the  crystals  often  appear  to  be  tetra- 
hedrally  hemihedral,  although  the 
etch  figures  on  cubic  faces  suggest 
hexoctahedral  symmetry  (Fig.  n). 
Octahedrons,  tetrahedrons,  icositet- 
rahedrons  and  combinations  of  these 
forms  are  common,  and  in  nearly  all 
cases  the  interfacial  edges  are  rounded 
and  the  crystal  faces  curved.  Some- 
times this  curving  is  so  pronounced 
that  the  individuals  are  practically 
spheres  (Fig.  12).  Twins  are  com- 
mon with  0(i  n)  as  the  twinning 
plane  (Fig.  13). 

The  cleavage  of  diamond  is  per- 
fect parallel  to  the  octahedral  face. 
This  is  an  important  characteristic,  as  the  lapidary  makes  use  of  it 
in  the  preparation  of  stones  for  cutting.  Its 
fracture  is  conchoidal.  Its  specific  gravity  is 
3.52  and  its  hardness  greater  than  that  of  any 
other  known  substance.  Most  diamonds  are 
dark  and  opaque,  or,  at  most,  translucent,  but 
many  are  found  that  are  transparent  and  color- 
less or  nearly  so.  Gray,  brown,  green,  yellow, 
blue  and  red  tinted  stones  are  also  known,  and, 
with  the  exception  of  the  blue  and  red  diamonds, 
these  are  more  common  than  the  colorless,  or 
so-called  white  stones.    The  luster  of  all  diamonds  is  adamantine,  and 


2. — Crystal  of  Diamond  with 
Rounded  Edges  and  Faces.    (Kraals.) 


ric.  13.— Octahedron  of 
Diamond  Twinned 
about  O(iii). 


INTRODUCTION— THE  ELEMENTS  39 

their  index  of  refraction  is  very  high,  «=  2.4024  for  red  rays,  2.4175  for 
yellow  rays,  and  2.4513  for  blue  rays.  In  consequence  of  their  strong 
dispersion,  the  reflection  of  light  from  the  inner  surfaces  of  transparent 
stones  is  very  noticeable,  causing  them  to  sparkle  brilliantly,  with  a 
handsome  play  of  colors.  It  is  this  latter  fact  and  the  great  hardness 
of  the  mineral  that  make  it  the  most  valuable  of  the  gems.-  The  mineral 
is  a  nonconductor  of  electricity. 

Three  varieties  of  the  diamond  have  received  distinct  names  in 
the  trade.     These  are: 

Gem  diamonds,  which  are  the  transparent  stones; 

Bart,  or  Bortz,  gray  or  black  translucent  or  opaque  rounded  masses, 
with  a  rough  exterior  and  the  structure  of  a  crystalline  aggregate;  and 

Carbonado,  black,  opaque  or  nearly  opaque  masses  possessing  a 
crystalline  structure,  but  no  distinct  cleavage. 

The  only  minerals  with  which  diamond  is  liable  to  be  confused 
are  much  softer,  and,  consequently,  there  is  little  difficulty  in  dis- 
tinguishing between  them. 

Syntheses. — Small  diamonds  have  been  made  by  fusing  in  an 
electric  furnace  metallic  iron  containing  a  small  quantity  of  carbon  and 
cooling  the  mass  suddenly  in  a  bath  of  molten  lead.  They  have  also 
been  made  by  heating  in  the  electric  arc  pulverized  carbon  on  a  spiral 
of  iron  wire  immersed  in  hydrogen  under  a  pressure  of  3,100  atmospheres. 
A  third  method,  which  resulted  in  the  production  of  tiny  octahedrons, 
consisted  in  melting  graphite  in  olivine,  or  in  a  mixture  of  silicates 
having  the  composition  of  the  South  African  "  blue  ground,"  with 
the  addition  of  a  little  metallic  aluminium  or  magnesium. 

Occurrence  and  Origin. — Diamonds  are  found  (1)  in  clay,  sand 
or  gravel  deposits  or  in  the  rocks  formed  by  the  consolidation  of  these 
substances,  where  they  are  associated  with  gold,  platinum,  topaz, 
garnet,  tourmaline  and  with  other  minerals  that  result  from  the  decom- 
position of  granitic  rocks,  (2)  in  a  basic  igneous  rock  containing  frag- 
ments of  shale  (a  consolidated  mud)  and  (3)  small  diamonds  have  been 
discovered  in  meteorites. 

The  manner  of  origin  of  diamonds  has  been  a  subject  of  contro- 
versy for  many  years.  The  most  popular  theory  ascribes  the  diamonds 
in  igneous  rocks  to  the  solution  of  organic  matter  in  the  rock  magmas 
and  the  crystallization  of  the  carbon  upon  cooling.  Another  theory 
regards  the  carbon  as  an  original  constituent  of  the  magma.  The 
diamonds  in  sand,  sandstone,  granite,  etc.,  are  believed  to  have  been 
transported  from  their  original  sources  and  deposited  in  river  channels 
or  on  beaches. 


40  DESCRIPTIVE  MINERALOGY 

Localities. — The  principal  localities  from  which  diamonds  are  obtained 
are  the  Madras  Presidency  in  India;  the  Province  of  Minas-Geraes  in 
Brazil;  the  Island  of  Borneo;  the  valleys  of  the  Vaal  and  Orange 
Rivers,  and  other  places  in  South  Africa,  and  the  valley  of  the  Mazaruni 
River  and  its  tributaries  in  British  Guiana.  Recently  diamond  fields 
have  been  discovered  in  New  South  Wales,  Australia,  in  the  valley  of 
the  Kasai  River  in  the  Belgian  Kongo,  in  Arkansas,  and  in  the  Tula- 
meen  district,  British  Columbia. 

In  the  United  States  a  few  gem  diamonds  have  been  found  from 
time  to  time  in  Franklin  and  Rutherford  counties  in  North  Carolina; 
in  the  gold-bearing  gravels  of  California,  and  in  soils  and  sands  in  the 
states  of  Alabama,  Virginia,  Wisconsin,  Indiana,  Ohio,  Idaho  and 
Oregon.  A  stone  (the  Dewey  diamond)  found  near  Richmond,  Virginia, 
a  few  years  ago  is  valued  at  $300  or  $400. 

The  principal  source  of  diamonds  and  carbonado  in  Brazil  at  the 
present  time  is  Bahia,  where  the  mineral  occurs  in  a  friable  sandstone 
along  river  courses.  The  output  of  this  region  has  decreased  so  greatly 
in  the  last  few  years  that  although  a  mass  of  carbonado  weighing  3,073 
carats  (the  largest  mass  of  diamond  material  ever  found)  was  obtained 
in  1895,  the  price  of  this  impure  diamond  rose  from  $10.50  per  carat 
in  1894  to  $36.00  per  carat  in  1896  and  $85.00  per  carat  for  the  best 
quality  in  19 16. 

The  only  diamond  field  of  prominence  in  the  United  States  is  that 
which  has  recently  been  exploited  near  Murfreesboro  in  Arkansas,  where 
the  conditions  are  similar  to  those  existing  in  South  Africa.  The  dia- 
monds occur  in  a  basic  igneous  rock  (peridotite)  that  cuts  through  sand- 
stones and  quartzites.  The  peridotite  is  weathered  to  a  soft  earth  or 
"  ground  "  in  which  the  diamonds  are  embedded.  Up  to  the  end  of 
1914  over  2,000  diamonds  had  been  found,  mostly  small  stones  weighing 
in  the  aggregate  550  carats,  valued  at  about  $12,000.  One,  however, 
weighed  8|  carats  and  another  7!  carats.  The  rough  unsorted  stones 
are  valued  at  $10  per  carat.     Three  stones  that  were  cut  were  found 

* 

to  be  worth  from  $60  to  $175  per  carat.  The  district  has  not  yet  been 
sufficiently  developed  to  prove  its  commercial  value.  The  diamonds 
in  British  Columbia  occur  in  the  same  kind  of  rock  as  those  in  Arkansas. 
The  few  that  have  thus  far  been  found  are  too  small  for  any  practical 
use. 

In  former  times  the  mines  of  India  and  Borneo  were  very  produc- 
tive, the  famous  Golconda  district  in  India  for  a  long  period  furnishing 
most  of  the  gems  to  commerce. 

The  African  mines  were  opened   in   1867.     Since  this  time  they 


INTRODUCTION— THE  ELEMENTS  41 

have  been  practically  the  only  producers  of  gem  material  in  the  world. 
It  is  estimated  that  the  quantity  of  uncut  diamonds  yielded  by  the 
mines  near  Kimberly  alone  have  amounted  in  value  to  the  enormous 
sum  of  $900,000,000.  The  output  of  the  African  mines  in  19 13  was 
sold  for  about  $53,000,000,  being  over  95  per  cent  of  the  world's  out- 
put of  gem  material.  Of  this  amount  about  $9,000,000  worth  of  stones 
were  furnished  by  German  Southwest  Africa,  the  balance  by  the 
Union  of  South  Africa.  The  diamonds  are  found  in  a  peridotite  which 
occurs  in  the  form  of  volcanic  necks,  or  "  pipes,"  cutting  carbonaceous 
shales.  The  igneous  rock  is  much  weathered  to  a  soft  blue  earthy  mass 
known  as  "  blue  earth."  Near  the  surface  where  exposed  to  the  action 
of  the  atmosphere  the  earth  is  yellow.  The  diamonds  are  scattered 
through  the  weathered  material  in  quantities  amounting  to  between 
.3  and  .6  carat  per  cubic  yard. 

Extraction. — Where  the  diamond  occurs  in  sand  and  gravel  it  is  ob- 
tained by  washing  away  the  lighter  substances. 

In  South  Africa  and  Arkansas  the  mineral  is  found  in  a  basic  volcanic 
rock  which  weathers  rapidly  on  exposure  to  the  air.  The  weathered 
rock  is  mined  and  spread  on  a  prepared  ground  to  weather.  When  suf- 
ficiently disintegrated  water  is  added  to  the  mass  and  the  mud  thus 
formed  is  allowed  to  pass  over  plates  smeared  with  grease.  The  dia- 
monds and  some  of  the  other  materials  adhere  to  the  grease,  but  most 
of  the  valueless  material  is  carried  off  by  the  water. 

Uses. — Transparent  diamonds  constitute  the  most  valuable  gems 
in  use.  Perfectly  white  stones,  or  those  possessing  decided  tints  of  red, 
rose,  green  or  blue  are  the  most  highly  prized.  They  are  sold  by 
weight,  the  standard  being  known  as  the  carat,  which,  until  recently, 
was  equivalent  to  3.168  grains  or  205  milligrams.  At  present  the  metric 
carat  is  in  almost  universal  use.  This  has  a  weight  of  200  milligrams. 
The  price  of  small  stones  depends  upon  their  color,  brilliancy  and  size — 
a  perfectly  white,  brilliant,  cut  stone  weighing  one  carat,  being  valued 
at  about  $175.00.  As  the  size  increases  the  value  increases  in  a  much 
greater  ratio,  the  price  obtained  for  large  stones  depending  almost  solely 
upon  the  caprice  of  the  purchaser. 

Nearly  all  the  gem  diamonds  put  upon  the  market  are  cut  before 
being  offered  for  sale.  The  chief  centers  of  diamond  cutting  are  Ant- 
werp and  Amsterdam  in  the  Old  World  and  New  York  in  America. 
The  favorite  cuts  are  the  brilliant  and  the  rose.  For  the  former  only 
octahedral  crystals,  or  those  that  will  yield  octahedrons  by  cleavage, 
are  used,  for  the  rose  cut  distorted  octahedrons  or  twinned  crystals. 
In  producing  the  "brilliant"  a  portion  of  the  top  of  an  octahedron  is  cut 


42 


DESCRIPTIVE  MINERALOGY 


A 


W=W 


A         A 

Bom 


Grown  Back,  ox  Parllion 

Step   or  Trap 


Grown 


gido  View 


ParCHon,  or  Base 

Brilliant 

Fig.  14. — Principal  "  cuts  "  of  Diamonds. 


off  and  a  small  portion  of  the  bottom.  On  the  remainder  are  cut  three 
or  four  bands  of  facets  running  horizontally  around  the  stone  (see  Fig.  14). 
The  "rose"  has  a  flat  base  surmounted  by  a  pyramidal  dome  consisting 
of  24  or  more  facets.  In  late  years  the  shapes  into  which  diamonds  are 
cut  have  been  determined  less  by  the  decrees  of  fashion  and  more  by  the 

/v— — 7v      desire  to  save  as  much  ma- 

>n>  vtI  mOi  /On  terial  as  possible,  and,  conse- 
quently, irregularly  shaped  cut 
diamonds  are  much  more 
common  than  formerly  (com- 
pare Fig.  17). 

Diamonds  are  employed 
also  as  cutting  tools.  Small 
fragments,  or  splinters  of  gem 
quality,  are  used  for  cutting 
and  polishing  diamonds  and 
other  gems,  and  small  crystals 
with  crystal  edges  for  cutting 
glass.  Small  cleavage  pieces 
are  utilized  in  the  manufacture  of  engravers'  tools  and  writing  instru- 
ments. Recently  diamonds  with  small  holes  of  from  .008  to  .0006  of  an 
inch  drilled  in  them,  have  been  employed  as  wire  dies. 

Bort  is  also  used  as  a  polishing  and  cutting  material,  while  carbonado, 
nearly  all  of  which  comes  from  Brazil,  is  used  in  the  manufacture  of 
boring  instruments.  Diamond  drills  consist  of  hollow  cylinders  of  soft 
iron  set  at  their  lower  edges  with  6,  8  or  1 2  black  diamonds.  By  rapid 
revolution  of  this  a  "core"  may  be  cut  from  the  hardest  rocks. 

Some  Famous  Diamonds. — The  largest  diamond  ever  found — the  Cull- 
inan — was  picked  up  at  the  Premier  Mine  (Fig.  15)  in  the  Transvaal  in 
January,  1905,  and  was  presented  to  King  Edward  of  England  as  a  birth- 
day gift  in  1908.  (Figs.  16  and  17.)  It  weighed  about  3,025  carats  (about 
1.37  pounds).  The  next  largest  was  found  in  June,  1893,  at  the  Jagers- 
fontein  mine.  It  is  known  as  the  Excelsior.  It  weighed  in  its  natural 
state  971  carats  and  was  3  inches  long  in  its  greatest  dimension.  It  was 
valued  at  $2,000,000.  It  is  said  to  have  been  presented  by  the  Presi- 
dent of  The  Orange  Free  State  to  Pope  Leo  XIII.  The  third  largest 
stone  is  the  Reitz.  It  is  a  640-carat  stone  found  at  the  same  mine  during 
the  close  of  1895.  This,  though  smaller,  is  said  to  be  handsomer  "than  the 
Excelsior.  The  most  noted  diamond  in  the  world  is  the  Kohinoor,  which 
weighed,  before  cutting,  186  carats.  It  is  now  a  brilliant  of  106  carats, 
belonging  to  the  crown  of  England.    Other  famous  diamonds  are  the 


INTRODUCTION— THE  ELEMENTS 


FlC.  15, — Premier  Diamond  Mines  in  South  Africa.     (After  WUiiem.) 


—The  Cullman  Diamond.     (Natural  size.) 


DESCRIPTIVE  MINERALOGY 


Fig.  17. — Gems  Cut  from  the  Cullinaii  Diamond.     [Two-fifths  nat.  size.) 

Orlov,  193  carats,  the  property  of  Russia;  the  Regent  or  Pitt  diamond 
of  137  carats  belonging  to  France;    the  Green  diamond  of  Dresden, 
weighing   48   carats,  and  the  Blue 
Hope  diamond,  weighing  44  carats. 
The  "  Star  of  the  South,"  found  in 
Brazil,  weighed  254  carats   before 
cutting   and    125    afterward.    The 
Victoria  diamond  from  one  of  the 
Kimberly  mines  weighed  457  carats 
when  found.    It  has  been  cut  to  a 
perfect  brilliant  of  180  carats  valued 
at  $1,000,000.     The   Tiffany   dia- 
mond (Fig.  18)  now  owned  in  New 
York    is   a   double  brilliant   of    a 
golden  yellow  color  weighing  128J 
F.G.l8.-The  Tiffany  Diamond.  (Nat-     carats  (25.702  grams)  and  valued  at 
ural  size.)    (Kindness  of  Tijjany  &■  Co.)     $100,000.      When  it  is  remembered 
that  a  five-carat  stone  is  large,  tie 
enormous  proportions  of  the  above-named  gems  are  better  appreciated. 

Graphite  (C) 

Graphite,  or  plumbago,  occurs  principally  in  amorphous  masses  of  a 
black,  clayey  appearance,  in.  radiated  masses,  in  brilliant  lead  black 
scales  or  plates,  and  occasionally  in  crystals  with  a  rhombohedral  habit. 

Like  diamond,  graphite  consists  of  carbon.  Crystals  from  Ceylon 
yield:  C=79.4o;  Ash=  15.50;  Volatile  matter=5.io.  The  mineral  is 
often  impure  from  admixture  with  clay,  etc. 


INTRODUCTION— THE  ELEMENTS  45 

« 

Crystals  of  the  material  a*e  so  rare  that  their  symmetry  is  still  in 
doubt.  Their  habit  is  hexagonal  (di trigonal  scalenohedral  class). 
Measurements  made  on  the  interfacial  angles  of  crystals  from  Ticon- 
deroga,  New  York,  gave  a  :  c=i  :  1.3859.  These  possess  a  rhombo- 
hedral  symmetry.  All  crystals  are  tabular  and  nearly  all  are  so  distorted 
that  the  measurements  of  their  interfacial  angles  cannot  be  depended 
upon  for  accuracy.  They  apparently  contain  the  planes  R(ioTi); 
oP(iooo);  00  P2(ii2o),  and  2P2(nIi). 

Graphite  is  black  and  earthy,  or  lustrous,  according  as  it  is  impure 
or  pure.  It  is  easily  cleavable  parallel  to  the  basal  plane,  and  the  cleav- 
age laminae  are  flexible.  It  is  very  soft,  its  hardness  being  only  1-2, 
its  density  about  2.25.  Its  luster  is  metallic  and  the  mineral  is  opaque 
even  in  the  thinnest  flakes.     It  is  a  conductor  of  electricity. 

Graphite  is  infusible  and  noncombustible  even  at  moderately  high 
temperatures.  Like  diamond,  however,  it  may  be  burned  under  cer- 
tain conditions  at  very  high  temperatures  (65o°-7oo°).  It  is  unaffected 
by  the  common  acids  and  is  not  acted  upon  by  the  atmosphere. 
When,  however,  it  is  subjected  to  the  action  of  strong  oxidizing  agents, 
such  as  a  warm  mixture  of  potassium  chlorate  (KCIO3)  and  fuming 
nitric  acid,  it  changes  to  a  yellow  substance  known  as  graphitic  acid 
(CiiH40r>).  It  is  thus  distinguished  from  amorphous  carbon,  like 
schungite  and  anthracite.  Moreover,  many  forms  of  graphite,  when 
moistened  with  fuming  nitric  acid  and  heated,  swell  up  and  send  out 
worm-like  processes.  Those  which  do  not  act  thus  are  called  graphitite. 
Natural  graphite  is  of  both  types. 

Its  color,  softness  and  infusibility  serve  to  distinguish  graphite  from 
all  other  minerals  but  molybdenite  (p.  75).  It  may  be  distinguished  from 
this  mineral  by  the  fact  that  it  contains  no  sulphur. 

Syntheses. — Crystalline  graphite  is  made  on  a  commercial  scale 
by  treating  anthracite  coal  or  coke  containing  about  5.75  per  cent  of 
ash  in  an  electric  furnace.  It  also  separates  when  molten  iron  con- 
taining dissolved  carbon  is  cooled. 

Occurrence  and  Origin. — Graphite  occurs  as  thin  plates  and  scales 
in  certain  igneous  rocks,  in  gneisses,  schists  and  limestones,  as  large 
scales  in  coarse  granite  dikes  (pegmatite)  and  in  crystalline  limestones, 
and  as  amorphous  masses  at  the  contacts  of  igneous  rocks  with  carbona- 
ceous rocks.  The  mineral  is  also  found  in  veins  cutting  sedimentary 
and  metamorphic  rocks.     Crystals  are  found  only  in  limestone. 

The  occurrence  of  graphite  in  sedimentary  and  igneous  rocks  sug- 
gests that  it  may  have  been  formed  in  several  ways.  It  is  thought 
that  the  material  in  limestone  and  quartz-schist  may  represent  carbo- 


46  DESCRIPTIVE  MINERALOGY 

naceous  material  that  was  deposited  with  the  sediments  and  which  has 
since  been  carbonized  by  heat  and  pressure.  The  material  in  peg- 
matite may  be  an  original  constituent  of  the  magma  that  produced  the 
rock,  and  the  graphite  may  be  the  product  of  pneumatolytic  processes; 
i.e.,  it  may  have  been  produced  by  deposits  from  vapors  that  accom- 
panied the  formation  of  the  pegmatite.  If  this  be  true,  the  mineral 
found  in  metamorphosed  limestone  and  schist  may  be  of  contact  origin ; 
i.e.,  it  may  have  been  produced  by  the  migration  of  gases  and  solutions 
from  igneous  rocks  into  the  mass  of  the  surrounding  sediments.  The 
vein  deoosits  probably  had  a  similar  origin,  the  mineral  having  been 
deposited  mainly  in  cracks  traversing  metamorphic  rocks.  On  the 
other  hand,  graphite,  in  some  instances,  appears  to  be  a  direct  separa- 
tion from  a  molten  magma. 

Localities. — The  principal  foreign  source  of  supply  for  commercial 
graphite  is  the  Island  of  Ceylon.  In  the  United  States  the  mineral  has 
been  mined  on  the  southeast  side  of  the  Adirondacks  in  New  York; 
in  Chester  County,  Pennsylvania;  near  Dillon,  Montana;  at  several 
points  in  Arkansas,  Georgia,  Alabama  and  North  Carolina;  in  Wyo- 
ming; in  Baraga  County,  Michigan,  and  to  a  small  extent  in  Colorado, 
Nevada,  and  Wisconsin.  It  occurs  also  abundantly  at  many  other 
places.  Its  chief  source  in  the  United  States  is  Graphite,  near  Lake 
George,  New  York. 

Preparation. — Graphite  is  obtained  on  a  commercial  scale  by  grind- 
ing the  rock  containing  it  and  floating  the  graphite  flakes. 

Uses. — Crude  graphite,  or  plumbago,  is  used  in  the  manufacture  of 
stove  and  other  polishes,  and  of  black  paint  for  metal  surfaces,  for  both 
of  which  it  is  especially  valuable  on  account  of  its  noncorroding  proper- 
ties. The  purified  mineral  is  mixed  with  clay  and  made  into  crucibles 
for  use  at  high  temperatures.  It  is  also  ground  and  used  in  this  form 
as  a  lubricant  for  heavy  machinery,  and  is  compressed  into  "  black  lead  " 
centers  for  lead  pencils. 

Production. — The  quantity  of  crude  graphite  mined  in  the  United 
States  during  191 2  amounted  to  2,445  tons,  valued  at  $207,033,  besides 
which  there  were  manufactured  6,448  tons,  valued  at  $830,193.  The 
imports  were  25,643  tons,  valued  at  $709,337. 

Schungite  is  a  black,  amorphous  carbon  with  a  hardness  of  3-4 
and  a  sp.gr.  of  1.981.  It  is  soluble  in  a  mixture  of  HNO3  and  KCIO3 
without  the  production  of  graphitic  acid.  It  occurs  in  some  crystalline 
schists. 


INTRODUCTION— THE  ELEMENTS  47 

SULPHUR  GROUP 

Sulphur  is  known  in  at  least  six  different  forms,  four  of  which  are 
crystalline.  The  two  best  known  forms  crystallize  respectively  in  the 
orthorhombic  (orthorhombic  bipyramidal  class)  and  the  monoclinic 
(prismatic  class)  systems.  The  former  separates  from  solutions  of  sulphur 
in  carbon  bisulphide  and  the  latter  separates  from  molten  masses. 
Both  the  orthorhombic  and  the  monoclinic  phases  are  believed  to  be 
formed  by  natural  processes,  but  the  latter  passes  over  into  the  former 
upon  standing,  so  that  its  existence  as  a  mineral  cannot  be  definitely 
proven.  Selenium  and  tellurium,  which  are  also  members  of  the  sul- 
phur group,  are  extremely  rare.  Tellurium  occurs  in  rhombohedral 
crystals  and  selenium  in  mixed  crystals  of  doubtful  character  with 
sulphur  and  tellurium. 

Sulphur  (S) 

Sulphur  occurs  in  nature  as  a  lemon-colored  powder,  as  spherical  or 
globular  masses,  as  stalactites  and  in  crystals. 

Chemically  it  is  pure  sulphur,  or  a  mixture  of  sulphur  and  clay, 


Fia.  19.  Fia.  20. 

Fig.  19. — Sulphur  Crystals  with  P,  in  (p)\  3P,  113  (5);  P«,  on  (»),  and  oP, 

001  (c). 

Fig.  20. — Distorted  Crystal  of  Sulphur.    (Forms  same  as  in  Fig.  19.) 

bitumen  or  other  impurities.  It  sometimes  contains  traces  of  tellu- 
rium, selenium  and  arsenic. 

Crystals  of  sulphur  are  usually  well  formed  combinations  of  ortho- 
rhombic bipyramids  and  domes,  with  or  without  basal  terminations. 
Their  axial  ratio  =.8108  :  1  :  1.9005.  The  principal  forms  observed 
are  P(ni),  P  66  (101),  Poo  (on),  JP(ii3)  and  oP(ooi)  (Figs.  19  and 
20).  The  habit  of  the  crystals  is  usually  pyramidal,  though  crystals 
with  a  tabular  habit  are  quite  common. 

Crystals  of  sulphur  are  yellow.    Their  streak  is  light  lemon  yellow. 


48  DESCRIPTIVE  MINERALOGY 

The  mineral  has  a  resinous  luster.  Its  hardness  is  only  1.5-2,  and 
density  about  2.04.  Its  fracture  is  conchoidal  and  cleavage  imper- 
fect. It  is  transparent  or  translucent,  is  brittle  and  is  a  non- 
conductor of  electricity.  Its  indices  of  refraction  for  sodium  light 
are  a=  1.9579,  0=  2.0377,  7=  2.2452. 

Massive  sulphur  varies  in  color  from  yellow  to  yellowish  brown 
greenish  gray,  etc.,  according  to  the  character  and  amount  of  impurities 
it  contains.  Its  powder  is  nearly  always  crystalline.  In  mass  it  pos- 
sesses a  lighter  color  than  the  crystals  or  the  massive  sulphur. 

At  a  temperature  of  1140  sulphur  melts,  and  at  2700  it  ignites, 
burning  with  a  blue  flame  and  evolving  fumes  of  SO2.  At  about  97 ° 
it  passes  over  into  the  monoclinic  phase.  It  is  insoluble  in  water  and 
acids,  but  is  soluble  in  oil  of  turpentine,  carbon  bisulphide  and  chlo- 
roform. 

There  are  few  minerals  that  are  apt  to  be  mistaken  for  sulphur. 
From  all  of  them  it  may  be  distinguished  by  its  brittleness  and  by  the 
fact  that  it  melts  readily  and  burns  with  a  nonluminous  blue  flame. 

Syntheses. — Crystals  with  the  form  of  the  mineral  are  produced  by 
the  evaporation  of  solutions  of  sulphur  in  carbon  bisulphide,  and  also 
by  sublimation  from  the  fumes  of  ore  roasters. 

Occurrence  and  Origin. — Sulphur  occurs  most  abundantly  in  regions 
of  active  or  extinct  volcanoes,  and  in  beds  associated  with  limestone 
and  gypsum  (CaS04-2H20).  In  volcanic  regions  it  is  produced  by 
reactions  between  the  gases  emitted  from  the  volcanoes,  or  by  the  reac- 
tions of  these  with  the  oxygen  of  the  air  (seep.  18).  The  deposits  in 
gypsum  beds  may  result  from  reduction  of  the  gypsum  by  organic 
matter.     Sulphur  is  formed  also  as  a  decomposition  product  of  sulphides. 

In  Iceland  and  other  districts  of  hot  springs  sulphur  is  often  deposited 
in  the  form  of  powder  as  the  result  of  reactions  similar  to  those  that 
take  place  between  the  gases  of  volcanoes.  These  hot  springs  are  always 
connected  with  dying  volcanoes,  being  frequently  but  the  closing 
stages  of  their  existence. 

Localities. — The  localities  at  which  sulphur  is  known  to  exist  are 
very  numerous.  Those  of  commercial  importance  are  Girgenti  in  Sicily, 
Cadiz  in  Spain,  Japan;  and  in  the  United  States,  at  the  geysers  of  the 
Napa  Valley,  Sonoma  County,  and  at  Clear  Lake,  Lake  County, 
California;  at  Cove  Creek,  Millard  County,  Utah;  at  the  mines  of  the 
Utah  Sulphur  Company  in  Beaver  County,  in  the  same  State;  at 
Thermopolis,  Wyoming,  and  at  various  hot  springs  in  Nevada.  The 
mineral  occurs  also  abundantly  in  the  Yellowstone  National  Park,  but 
cannot  be  placed  on  the  market  because  of  high  transportation  charges. 


INTRODUCTION— THE  ELEMENTS  49 

Its  principal  occurrence  in  the  United  States  is  at  Lake  Charles  in 
Calcasieu  Parish,  La.,  where  it  impregnates  a  bed  of  limestone  at 
a  depth  of  from  450  to  1,100  feet.  It  occurs  also  abundantly  in  the 
coastal  districts  of  Texas.    Here  it  is  associated  with  gypsum. 

Extraction. — Sulphur,  when  mined,  is  mixed  with  clay,  earth,  rock  and 
other  impurities.  Until  recently  it  was  purified  by  piling  in  heaps  and 
igniting.  A  portion  of  the  sulphur  burned  and  melted  the  balance, 
which  flowed  off  and  was  caught.  A  purer  product  is  obtained  by  dis- 
tillation. "Flowers  of  Sulphur"  are  made  in  this  way.  At  present 
much  of  the  sulphur  is  extracted  by  treating  the  impregnated  rock  in 
retorts  with  steam  under  a  pressure  of  60  pounds  and  at  a  temperature 
of  1440  C.  The  sulphur  melts  and  flows  to  the  bottom  of  the  retorts 
from  which  it  is  drawn  off. 

In  Louisiana  and  Texas,  superheated  steam  is  forced  downward  into 
the  sulphur-impregnated  rocks.  This  melts  the  sulphur,  which  con- 
stitutes about  70  per  cent  of  the  rock  mass.  The  melted  sulphur  is 
forced  to  the  surface  and  caught  in  wooden  bins.  The  crude  material 
has  a  guaranteed  content  of  over  99!  per  cent  sulphur. 

Uses. — Sulphur,  or  brimstone,  is  used  in  the  manufacture  of  some 
kinds  of  matches,  in  making  gunpowder,  and  in  vulcanizing  rubber 
to  increase  its  strength  and  elasticity.  It  is  used  extensively  in  the 
manufacture  of  sulphuric  acid,  but  is  rapidly  giving  way  to  pyrite 
for  this  purpose.  It  is  also  utilized  for  bleaching  straw,  in  the  man- 
ufacture of  certain  pigments,  among  which  is  vermilion,  and  in  the 
preparation  of  certain  medicinal  compounds. 

Production. — Most  of  the  domestic  product  is  at  present  from  the 
Calcasieu  Parish,  La.,  where  about  300,000  tons  are  mined  annually. 
New  mines  have  been  opened  near  Thermopolis  in  Wyoming,  in  Bra- 
zoria County,  Texas,  and  at  Sulphur  Springs,  Nevada.  The  total 
amount  of  the  mineral  mined  in  191 2  was  303,472  tons,  valued  at  $5,256,- 
422.  Besides,  there  were  imported  about  29,927  tons  valued  at  $583,974, 
most  of  which  came  from  Japan.  Sicily  is  the  largest  producer  of  the 
mineral,  extracting  about  400,000  tons  annually. 

ARSENIC   GROUP 

The  arsenic  group  comprehends  metallic  arsenic,  antimony,  bismuth 
and  (according  to  some  mineralogists),  tellurium,  besides  compounds 
of  these  metals  with  each  other.  They  all  crystallize  in  the  rhombo- 
hedral  division  of  the  hexagonal  system  (di trigonal  scalenohedral  class). 
The  only  members  of  the  group  that  are  at  all  common  are  arsenic  and 
antimony. 


60  DESCRIPTIVE  MINERALOGY 

Arsenic  (As) 

Arsenic  is  rarely  found  in  crystals.  It  usually  occurs  massive  or  in 
botryoidal  or  globular  forms. 

Specimens  of  the  mineral  are  rarely  pure.  They  usually  contain 
some  antimony,  and  traces  of  iron,  silver,  bismuth,  and  other  metals. 

The  crystals  are  cubical  in  habit,  with  an  axial  ratio  of  i  :  1.4025. 
The  principal  forms  observed  are:  oR(oooi),  R(ioTi),  JR(ioT4), 
—  JR(oiT2)  and  —^(0332).  Twins  are  rare,  with  —  JR(oiT2)  the 
twinning  plane. 

Arsenic  is  lead-gray  or  tin-white  on  fresh  fractures,  and  dull  gray  or 
nearly  black  on  surfaces  that  have  been  exposed  for  some  time  to  the 
atmosphere. 

Crystals  cleave  readily  parallel  to  the  base.  The  fracture  of  massive 
pieces  is  uneven.  The  mineral  is  brittle.  Its  hardness  is  3.5  and  its 
density  5.6-5.7.  Its  streak  is  tin- white  tarnishing  soon  to  dark  gray. 
It  is  an  electrical  conductor. 

Arsenic  may  easily  be  distinguished  from  nearly  all  other  minerals, 
except  antimony  and  some  of  the  rarer  metals,  by  the  color  of  its  fresh 
surfaces.  From  these,  with  the  exception  of  antimony,  it  is  also  readily 
distinguished  by  its  action  on  charcoal  before  the  blowpipe,  when  it 
volatilizes  completely  without  fusing,  at  the  same  time  tingeing  the 
flame  blue  and  giving  rise  to  dense  white  fumes  of  AS2O3,  which  coat  the 
charcoal.  The  fumes  of  arsenic  possess  a  very  disagreeable  and  oppres- 
sive odor,  while  those  of  antimony  have  no  distinct  odor. 

Syntheses. — Arsenic  has  been  obtained  in  crystals  by  subliming 
arsenic  compounds  protected  from  the  air.  It  has  also  been  obtained  in 
the  wet  way  by  heating  realgar  (AS2S3)  with  sodium  bicarbonate  at 
3oo°C. 

Occurrence  and  Origin. — Arsenic  often  accompanies  ores  of  antimony, 
silver,  lead  and  other  metals  in  veins  in  crystalline  rocks,  especially  in 
their  upper  portions,  where  it  was  formed  by  reduction  from  its  com- 
pounds. 

Localities. — The  silver  mines  at  Freiberg  and  other  places  in  Saxony 
afford  native  arsenic  in  some  quantity.  It  is  found  also  in  the  Harz;  at 
Zmeov  in  Siberia;  in  the  silver  mines  of  Chile  and  elsewhere. 

Within  the  boundaries  of  the  United  States  arsenic  occurs  only  in 
small  quantity  at  Haverhill,  N.  H.,  at  Greenwood,  Me.,  and  at  a  silver 
and  gold  mine  near  Leadville,  Colo. 

Uses. — Arsenic  is  used  only  in  the  forms  of  its  compounds.  The 
native  metal  occurs  too  sparingly  to  be  of  commercial  importance. 


INTRODUCTION— THE  ELEMENTS  51 

Most  of  the  arsenic  compounds  used  in  commerce  are  obtained  from 
smelter  fumes  produced  by  smelting  arsenical  copper  and  gold  ores. 

Antimony  (Sb) 

Antimony  is  more  common  than  arsenic,  which  it  resembles  in  many 
respects.  It  is  generally  found  in  lamellar,  radial  and  botryoidal  masses, 
though  rhombohedral  crystals  are  known. 

Most  antimony  contains  arsenic  and  traces  of  silver,  lead,  iron  and 
other  metals. 

Its  crystals  are  rhombohedral  or  tabular  in  habit,  and  have  an  axial 
ratio  of  a  :  c=i  :  1.3236.  The  forms  observed  on  them  are  the  same 
as  those  on  arsenic  with  the  addition  of  ooP2(ii2o),  and  several 
rhombohedrons.  Twinning  is  often  repeated.  The  cleavage  is  perfect 
parallel  to  oP(oooi). 

Antimony  exhibits  brilliant  cleavage  surfaces  with  a  tin-white  color. 
On  exposed  surfaces  the  color  is  dark  gray.  The  mineral  differs  from 
arsenic  in  its  greater  density  which  is  6.65-6.72,  and  in  the  fact  that  it 
melts  (at  6290)  before  volatilizing.  Its  fumes,  moreover,  are  devoid  of 
the  garlic  odor  of  arsenic  fumes. 

Syntheses. — Crystals  of  antimony  are  often  obtained  from  the  flues  of 
furnaces  in  which  antimonial  lead  is  treated.  They  have  also  been 
made  by  the  reduction  of  antimony  compounds  by  hydrogen  at  a  high 
temperature. 

Occurrence  and  Localities, — Antimony  occurs  in  lamellar  concretions 
in  limestone  near  Sala,  Sweden,  and  at  nearly  all  of  the  arsenic  localities 
mentioned  above,  especially  in  veins  containing  stibnite  (Sb2S3)  or  silver 
ores.  It  is  found  also  in  fairly  large  quantities  in  veins  near  Fredericton, 
York  County,  New  Brunswick,  in  California  and  elsewhere. 

Uses. — Although  the  metal  antimony  is  of  considerable  importance 
from  an  economic  point  of  view,  being  used  largely  in  alloys,  the  native 
mineral,  on  account  of  its  rarity,  enters  little  into  commerce.  Some  of 
the  antimony  used  in  the  arts  is  produced  from  its  sulphide,  stibnite 
(see  p.  72).  Most  of  the  metal,  however,  is  obtained  in  the  form  of  a 
lead-antimony  alloy  in  the  smelting  of  lead  ores  and  the  refining  of  pig 
lead. 

Bismuth  (Bi)  is  usually  in  foliated,  granular  or  arborescent  forms, 
and  very  rarely  in  rhombohedral  crystals,  with  a  :  c=  1  :  1.3036.  It  is 
silver-white  with  a  reddish  tinge,  is  opaque  and  metallic.  Its  streak  is 
white,  its  hardness  2-2.5  and  density  9.8.  It  fuses  at  271  °.  On  charcoal 
it  volatilizes  and  gives  a  yellow  coating.    It  dissolves  in  HNO3.     When 


52  DESCRIPTIVE  MINERALOGY 

this  solution  is  diluted  a  white  precipitate  results.  The  mineral  occurs 
in  veins  with  ores  of  silver,  cobalt,  lead  and  zinc.  It  is  of  no  commercial 
importance.  Most  of  the  metal  is  obtained  in  the  refining  of  lead.  In  1913 
the  United  States  produced  185,000  lbs.  and  Bolivia  about  606,000  lbs. 

Tellurium  (Te)  usually  occurs  in  prismatic  crystals  with  a  tin-white 
color  and  in  finely  granular  masses  in  veins  of  gold  and  silver  ores, 
especially  sulphides  and  tellurides.  Its  hardness  is  2  and  density  6.2. 
Before  the  blowpipe  it  fuses,  colors  the  flame  green,  coats  the  charcoal 
with  a  white  sublimate  bordered  by  red,  and  yields  white  fumes. 

The  mineral  tellurium  is  of  little  value  as  a  source  of  the  metal. 
Most  of  that  used  in  the  arts  is  obtained  as  a  by-product  in  the  elec- 
trolytic refining  of  copper  made  from  ores  containing  tellurides  and 
from  the  flue  dust  of  acid  chambers  and  smelting  furnaces.  The  United 
States,  in  1913,  produced  about  10,000  lbs.  of  tellurium  and  selenium, 
valued  at  $35,000. 

THE  METALS 

The  metallic  elements  occur  as  minerals  in  comparatively  small  quan- 
tity, most  of  the  metals  used  in  the  industries  being  obtained  from  their 
compounds.  Iron,  the  most  common  of  all  the  metals  used  in  com- 
merce, is  rare  as  a  mineral,  as  are  also  lead  and  tin.  Silver,  copper,  gold 
and  platinum  are  sufficiently  important  to  be  included  in  our  list  for 
study.  Gold  and  platinum  are  known  almost  exclusively  in  the  metallic 
state.  A  large  portion  of  the  copper  produced  in  this  country  is  also 
native,  and  some  of  the  silver. 

Silver,  copper,  lead,  gold,  mercury  and  the  alloys  of  gold  and  mer- 
cury crystallize  in  distinct  crystals  belonging  to  the  isometric  system 
(hexoctohedral  class).  Platinum,  as  usually  found,  is  in  small  plates 
and  grains.  Crystals,  however,  have  been  described  and  they,  too,  are 
isometric.  Platinum  and  iron  are  separated  from  the  other  metals  and, 
together  with  the  rare  alloys  of  platinum  with  iridium  and  osmium,  are 
placed  in  a  distinct  group  which  is  dimorphous.  The  reason  for  this  is 
that  platinum,  although  isometric  in  crystallization,  often  contains 
notable  traces  of  iridium,  which  in  its  alloy  with  osmium  is  hexagonal 
(rhombohedral).  Iridium,  thus,  is  dimorphous,  hence  platinum  which 
forms  crystals  with  it  and  is,  therefore,  isomorphous  with  it,  must  also 
be  regarded  as  dimorphous.  The  various  platinum  metals  thus  com- 
prise an  isodimorphous  group.  Iron  is  placed  in  the  same  group  because 
it  is  so  frequently  alloyed  with  platinum.  The  metals  are,  therefore, 
divisible  into  two  groups,  one  of  which  comprises  the  metals  named  at 


INTRODUCTION— THE  ELEMENTS 


53 


the  beginning  of  this  paragraph  and  the  other  consists  of  the  rare  metals, 
palladium,  platinum,  iridium,  osmium,  iron  and  their  alloys.  The 
metal  tin,  which  is  tetragonal  in  its  native  condition,  constitutes  a  third 
group,  but  since  it  is  extremely  rare  it  will  not  be  referred  to  again. 

GOLD  GROUP 

This  group  embraces  the  native  metals,  copper,  silver,  goli,  gold- 
amalgam  (Au-Hg),  silver -amalgam  (Ag-Hg),  mercury,  and  leal.  All 
crystallize  in  the  isometric  system  (hexoctahedral  class),  and  all  form 
twins,  with  O(m)  the  twinning  plane.  Copper,  silver  and  gold  are 
the  most  important. 

Copper  (Ctt) 

Most  of  the  copper  of  commerce  is  obtained  from  one  or  the  other  of 

its   sulphides.     A    large   portion,   however,   is 

found  native.     This  occurs  in  tiny  grains  and 

flakes,    in    groups   of   crystals   and    in    large 

masses  of  irregular  shapes. 

In   spite  of  its  softness  copper  is  better 

crystallized  than  either  gold  or  silver.     It   is 

true  that  its  crystals  are  usually  flattened  and 

Otherwise  distorted,  but,  nevertheless,    planes 

can  very  frequently  be  detected  upon   them. 

The  principal  forms  observed  arc  °o  O  °o  (ioo), 

oo  O(no),  O(ni),  and  various  tctrahexahedra 

and  kositetrahedra.  (Figs.  21  and  22.)  Some- 
times the  crystals  are  sim- 
ple, in  other  cases  they  are 
twinned  parallel  to  O. 
Often  they  are  skeleton 
crystals.  Groups  of  crys- 
tals are  very  common. 
These  possess  the  arbo- 
rescent forms  so  frequently 
seen  in  specimens  from 
Keweenaw  Point  in  Mich- 
igan, or  are  groupings  of 
simple  forms  extended  in 
the  direction  of  the  cubic 
axes. 
Copper  is  very  ductile  and  very  malleable.    Its  hardness  is  only 


Fio.  2i.— Copper  Crystal 
with    »  O,  no  (d)  and 


aa. — Crystal  of  Copper  from  Keweenaw  Point, 
Mich.,  with  =o0{no)  and  202(211). 


54  DESCRIPTIVE  MINERALOGY 

2.5-3  and  its  density  about  8.8.  It  possesses  no  cleavage,  and  its  frac- 
ture, like  that  of  the  other  metals,  is  hackly.  In  color  it  is  copper-red 
by  reflected  light,  often  tarnishing  to  a  darker  shade  of  red.  In  very 
thin  plates  it  is  translucent  with  a  green  color.  The  metal  fuses  at 
1083 °  and  easily  dissolves  in  acids.  It  is  an  excellent  conductor  of  elec- 
tricity. 

Its  most  characteristic  chemical  reaction  is  its  solubility  in  nitric 
acid  with  the  evolution  of  brownish  red  fumes  of  nitrous  oxide  gas. 

Copper  may  easily  be  distinguished  from  all  other  substances  except 
gold  and  a  few  alloys  by  its  malleability  and  color.  It  is  distinguished 
from  gold  by  the  color  of  its  borax  bead  and  by  its  solubility  in  nitric 
acid  with  the  production  of  a  blue  solution  which  takes  on  an  intense 
azure  color  when  treated  with  an  excess  of  ammonia.  From  the  alloys 
that  resemble  it,  copper  may  be  distinguished  by  its  greater  softness  and 
the  fact  that  it  yields  no  coatings  when  heated  on  charcoal,  while  at  the 
same  time  its  solution  in  nitric  acid  yields  the  reaction  described  above. 

Syntheses. — Copper  crystals  separate  upon  cooling  solutions  of  the 
metal  in  silicate  magmas  and  upon  the  electrolysis  of  the  aqueous  solu- 
tions of  its  salts. 

Occurrence. — The  principal  modes  of  occurrence  of  the  metal  are,  (1) 
as  fine  particles  disseminated  through  sandstones  and  slates,  (2)  as  solid 
masses  filling  the  spaces  between  the  pebbles  and  boulders  making  up 
the  rock  known  as  conglomerate,  (3)  in  the  cavities  in  old  volcanic  lavas, 
known  as  amygdaloid,  (4)  as  crystals  or  groups  of  crystals  imbedded  in 
the  calcite  of  veins,  (5)  in  quartz  veins  cutting  old  igneous  rocks  or 
schists,  and  (6)  associated  with  the  carbonates,  malachite  and  azurite, 
and  with  its  different  sulphur  compounds,  in  the  weathered  zone  of 
many  veins  of  copper  ores. 

The  copper  that  occurs  in  the  upper  portions  of  veins  of  copper 
sulphides  is  plainly  of  secondary  origin.  That  which  occurs  in  conglom- 
erates and  other  fragmental  rocks  and  in  amygdaloids  was  evidently 
deposited  by  water,  but  whether  by  ascending  magmatic  water  or  by 
descending  meteoric  water  is  a  matter  of  doubt. 

Localities. — Native  copper  is  found  in  Cornwall,  England,  in  Nassau, 
Germany,  in  Bolivia,  Peru,  Chile  and  other  South  American  countries, 
in  the  Appalachian  region  of  the  United  States  and  in  the  Lake  Superior 
region,  both  on  the  Canadian  and  the  American  sides. 

The  most  important  district  in  the  world  producing  native  copper  is 
on  Keweenaw  Point,  in  Michigan.  The  mineral  occurs  mainly  in  a  bed 
of  conglomerate  of  which  it  constitutes  from  1  to  3  per  cent,  though  it  is 
found  abundantly  also  in  sandstone  and  in  the  amygdaloidal  cavities 


INTRODUCTION— THE  ELEMENTS  55 

of  lavas  associated  with  the  conglomerates.  Veins  of  calcite,  through 
which  groups  of  bright  copper  crystals  are  scattered  are  also  very  plentiful 
in  many  parts  of  the  district.  The  copper  is  nearly  always  mixed  with 
silver  in  visible  grains  and  patches. 

Extraction  and  Refining. — The  rock  containing  the  native  metal  is 
crushed  and  the  metal  is  separated  from  the  useless  material  by  wash- 
ing. The  concentrates,  consisting  of  the  crushed  metal  mixed  with 
particles  of  rock  and  other  impurities  are  then  refined  by  smelting 
methods  or  by  electrolysis. 

Uses. — The  uses  of  copper  are  so  many  that  all  of  even  the  important 
uses  cannot  be  mentioned  in  this  place.  Both  as  a  metal  and  in  the  form 
of  its  alloys  it  has  been  employed  for  utensils  and  war  implements  since 
the  earliest  times.  In  recent  times  one  of  its  principal  uses  has  been  for 
the  making  of  telegraph,  telephone  and  trolley  wires.  It  is  employed 
extensively  in  electroplating  by  all  the  great  newspapers  and  publishers, 
and  is  an  important  constituent  of  the  valuable  alloys  brass,  bronze, 
bell  metal  and  German  silver.  Its  compound,  blue  vitriol  (copper  sul- 
phate), is  used  in  galvanic  batteries,  and  its  compounds  with  arsenic 
are  utilized  as  pigments. 

Production. — The  world's  production  of  copper  amounted  to  1,126,- 
000  tons  in  191 2,  but  a  large  portion  of  this  was  obtained  from  its  car- 
bonates and  sulphides.  The  quantity  obtained  from  the  native  metal  is 
unknown.  The  contribution  of  the  United  States  to  this  total  was 
about  621,000  tons,  valued  at  about  $206,382,500,  of  which  115,000  tons 
was  native  copper  from  the  Lake  Superior  region.  The  largest  single 
mass  ever  found  in  the  Lake  Superior  region  weighed  420  tons. 

Silver  (Ag) 

Silver  is  usually  found  in  irregular  masses,  in  flat  scales,  in  fibrous 
clusters,  and  in  crystal  groups  with  arborescent  or  acicular  forms. 
Sometimes  the  crystals  are  well  developed,  more  frequently  they  ex- 
hibit only  a  few  distinct  faces,  but  in  most  cases  they  are  so  distorted 
that  it  is  difficult  to  make  out  their  planes. 

Pure  silver  is  unknown.  The  mineral  as  it  is  usually  obtained  con- 
tains traces  of  gold,  copper,  and  often  some  of  the  rarer  metals,  depend- 
ing upon  its  associations. 

Ideally  developed  silver  crystals  are  rare.  They  usually  show 
00  O  00  (100),  00  0(i  10),  0(i  11)  various  tetrahexahedrons  and  other 
more  complicated  forms.  The  majority  of  the  crystals  are  distorted  by 
curved  faces  and  rounded  edges,  and  many  of  them  by  flattening  or 


56  DESCRIPTIVE  MINERALOGY 

elongation.  The  arborescent  groups  usually  branch  at  angles  of  6o°, 
one  of  the  characteristic  angles  for  groups  of  isometric  crystals.  Twins 
are  quite  common,  with  O(in)  the  twinning  plane. 

Silver  is  a  white,  metallic  mineral  when  its  surfaces  are  clean  and 
fresh.  As  it  usually  occurs  it  possesses  a  gray,  black  or  bluish  black 
tarnish  which  is  due  to  the  action  of  the  atmosphere  or  of  solutions. 
The  tarnish  is  commonly  either  the  oxide  or  the  sulphide  of  silver. 

The  mineral  has  no  cleavage.  Its  fracture  is  hackly.  It  is  soft 
(hardness  2-3),  malleable  and  ductile,  and  is  an  excellent  conductor 
of  heat  and  electricity.  Its  density  is  about  10.5,  varying  slightly 
with  the  character  and  abundance  of  its  impurities.  It  fuses  at 
9600. 

It  is  readily  soluble  in  nitric  acid  forming  a  solution  from  which 
a  white  curdy  precipitate  of  silver  chloride  is  thrown  down  on  the 
addition  of  any  chloride:  This  precipitate  is  easily  distinguished  from 
the  corresponding  lead  chloride  by  its  insolubility  in  hot  water. 

Synthesis. — Crystals  bounded  by  O(in)  and  00  O  00  (100)  have  been 
made  by  the  reduction  of  silver  sulphate  solutions,  with  sulphurous 
acid. 

Occurrence. — Native  silver  is  found  in  veins  with  calcite  (CaCOs), 
quartz  (SiCfe),  and  other  gangues  traversing  crystalline  rocks,  like 
granite  and  various  lavas,  and  also  in  veins  cutting  conglomerates 
and  other  rocks  formed  from  pebbles  and  sands.  It  is  also  disseminated 
in  small  particles  through  these  rocks.  It  occurs  invisibly  disseminated 
in  small  quantities  through  many  minerals,  particularly  sulphides, 
and  visibly  intermingled  with  native  copper.  It  is  abundant  in  the  upper 
weathered  zones  of  many  veins  of  silver-bearing  ores,  and  in  the  zones 
of  secondary  enrichment  in  the  same  veins.  It  also  occurs  in  small 
quantity  in  placers.  In  general,  its  origin  is  similar  to  that  of  gold 
(see  p.  59). 

Localities. — The  localities  in  which  silver  is  found  are  too  numerous 
to  mention.  Andreasberg  in  the  Harz  has  produced  many  fine  crys- 
tallized specimens.  The  principal  deposits  now  worked  are  at  Cobalt 
in  Canada,  in  Peru,  in  Idaho,  at  Butte,  Montana,  in  Arizona  and  at 
many  places  in  Colorado.  On  Keweenaw  Point,  in  Michigan,  fine 
crystals  have  been  found  in  the  calcite  veins  cutting  the  copper-bearing 
rocks,  and  masses  of  small  size  in  the  native  copper  so  abundant  in  the 
district.  Indeed  some  of  the  copper  is  so  rich  in  silver  that  the  ore 
was  in  early  times  mined  almost  exclusively  for  its  silver  content.  At 
present  the  silver  is  recovered  from  the  copper  in  the  refining  process. 
At  Cobalt  the  mineral  occurs  in  well  defined  veins  one  inch  to  one  foot 


INTRODUCTION— THE   ELEMENTS  57 

or  more  in  width,  cutting  a  series  of  slightly  inclined  pre-Cambrian 
beds  of  fragmental  and  igneous  rocks.  The  veins  contain  native  silver, 
sulphides  and  arsenides  of  cobalt,  nickel,  iron  and  copper,  calcite  and  a 
little  quartz.  Many  of  the  veins  are  so  rich  (Fig.  23)  that  Cobalt  has 
become  one  of  the  most  important  camps  producing  native  silver  in 
the  world. 

Extraction  and  Refining. — Silver  is  obtained  from  placers  in  small 
quantity  by  the  methods  made  use  of  in  obtaining  gold  (see  p.  61), 
i.e.,  by  hydraulic  mining.  When  it  occurs  in  quartz  veins  or  in  complex 
ores  such  as  constitute  the  oxidized  portion  of  ore-bodies,  the  mass 
may  be  crushed  and  then  treated  with  quicksilver,  which  amalgamates 
with  the  native  silver  and  gold,  forming  an  alloy.    Such  ores  are  known 


FlG.   33.— Plate   of  Silver  from   Conlanas  Mine,   Cobalt. 

ins.    Weight  37  lbs.     {Photo  by  C,  W.  Knight.) 

as  free  milling.  The  silver  is  freed  from  the  gold  and  other  metals  by 
a  refining  process.  It  is  separated  from  native  copper  by  electrolytic 
methods. 

Uses. — Silver  Is  used  in  the  arts  to  a  very  large  extent.  Jewelry, 
ornaments,  tableware  and  other  domestic  utensils,  chemical  apparatus 
and  parts  of  many  physical  instruments  are  made  of  it.  It  is  used  also 
in  the  production  of  mirrors  and  in  the  manufacture  of  certain  compounds 
used  in  surgery  and  in  photography.  Its  alloy  with  copper  forms  the 
staple  coinage  of  China,  Mexico  and  most  of  the  South  American  coun- 
tries, and  the  subsidiary  (or  small)  coinage  of  most  countries.  In 
the  United  States  it  is  used  in  the  coinage  of  silver  dollars  and  of  frac- 
tions of  the  dollar  as  small  as  the  dime.  The  silver  coins  of  the  United 
States  are  nine-tenths  silver  and  one-tenth  copper,  the  latter  metal  being 
added  to  give  hardness.     English  coins  contain  12$  parts  silver  to  one 


58  DESCRIPTIVE  MINERALOGY 

part  of  copper.    In  1912  the  world's  coinage  of  silver  consumed  161,- 
763,415  oz.,  with  a  value  after  coinage  of  $171,293,000. 

Production. — The  total  production  of  silver  in  the  United  States 
during  1912  was  over  63,766,000  oz.,  valued  at  over  $39,197,000,  of 
which  about  $100,000  worth  came  from  placers  and  $325,000  worth 
from  tie  copper  mines  of  Michigan.  The  balance  was  obtained  by 
smelting  silver  compounds  and  in  the  refining  of  gold,  lead,  copper  and 
zinc  ores.  The  world's  production  of  silver  during  1912  was  224,488,- 
000  oz.,  valued  at  over  $136,937,000,  but  most  of  this  was  obtained 
from  the  compounds  of  silver  and  not  from  the  native  metal.  The 
proportion  obtained  from  the  mineral  is  not  definitely  known;  but  the 
production  of  Canada  was  more  than  30,243,000  oz.,  valued  at 
$17,672,000  and  nearly  all  of  this  came  from  Cobalt,  where  the  ore  is 
native  silver. 

Gold  (Au) 

A  large  portion  of  the  gold  of  the  world  has  been  obtained  in  the 
form  of  native  metal.    The  greater  portion  of  the  metal  is  so  very  finely 
disseminated  through  other  minerals  that  no  sign  of  its  presence  can  be 
detected  even  with  high  powers  of  the  microscope.     Although  present 
in  such  minute  quantities  it  is  very  widely  spread,  many  rocks  con- 
taining it  in  appreciable  quantities.     Its  visible  grains,  as  usually  found, 
are  little  rounded  particles  or  thin  plates  or 
scales  mixed   with  sand  or  gravel,   or  tiny 
irregular  masses  scattered  through  white  vein- 
quartz. 

Native  gold  rarely  occurs  in  well  formed 
crystals.  The  metal  is  so  soft  that  its  crystals 
are  battered  and  distorted  by  very  slight 
pressure.  Occasionally  well  developed  crys- 
tals, bounded  by  octahedral,  dodecahedral 
Fie.  14 -Octahedwl  Skde-  and  complicated  icositetrahedral  and  tetra- 
ton   Crystal  of  Gold  with     ,         ,     ,     ,     ,  .  ,      , 

Etched  Faces  hexahedral    faces  are  met   with,  but  usually 

the  crystals  are  elongated  or  flattened.  Skele- 
ton crystals  (Fig.  24)  and  groups  of  crystals  are  more  frequently  found 
than  are  simple  crystals.  Twins  are  common,  with  0(m)  the  twin- 
ning plane. 

As  found  in  nature,  gold  is  frequently  alloyed  with  silver  and  it 
often  contains  traces  of  iron  and  copper  and  sometimes  small  quanti- 
ties of  the  rarer  metals. 

Gold  containing  but  a  trace  of  silver  up  to  16  per  cent  of  this  metal 


INTRODUCTION— THE  ELEMENTS  59 

is  known  simply  as  gold.  When  the  percentage  of  silver  present  is 
larger  it  is  said  to  be  argentiferous.  When  the  percentage  reaches 
20  per  cent  cr  above  the  alloy  is  called  clectrun.  Palladium,  rhodium 
and  bismuth  gold  are  alloys  of  the  last-named  metal  with  the  rare  metals 
palladium  or  rhodium  or  with  the  more  common  bismuth. 

The  color  of  the  different  varieties  of  the  mineral  varies  from  pinkish 
silver-white  to  almost  copper-red.  Pure  gold  is  golden  yellow.  With 
increase  cf  silver  it  becomes  lighter  in  color  and  with  increase  in  copper, 
darker.  The  rich  red-yellow  of  much  of  the  gold  used  in  the  arts  is  due 
to  the  admixture  of  copper.  In  very  thin  plates  or  leaves  (.001  mm.) 
gold  is  translucent  with  a  blue  or  green  tint. 

Gold  is  soft,  malleable  and  ductile.  Its  luster  is,  of  course,  metallic 
and  its  streak,  yellow.  When  pure  its  density  is  19.43,  its  hardness 
between  2  and  3,  and  its  fusing  point  10620.  The  metal  is  insoluble  in 
most  acids,  but  it  is  readily  dissolved  in  a  mixture  of  nitric  and  hydro- 
chloric acids  (aqua  regia).  It  is  not  acted  upon  by  water  or  the  atmos- 
phere. Its  negative  properties  distinguish  it  from  the  other  substances 
which  it  resembles  in  appearance.     It  is  a  good  conductor  of  electricity. 

Syntheses. — Crystals  of  gold  have  been  obtained  by  heating  a  solu- 
tion of  AuCfe  in  amy!  alcohol,  and  by  treating  an  acid  solution  of  the 
same  compound  with  formaldehyde. 

Occurrence. — Native  gold  is  found  in  the  quartz  of  veins  cutting 
through  granite  and  schistose  rocks,  or  in  the  gravels  and  sands  of  rivers 
whose  channels  cut  through  these,  and  in  the  sands  of  beaches  bordering 
gold-producing  districts.  It  is  sometimes  found  in  the  compacted 
gravels  of  old  river  beds,  in  a  rock  known  as  conglomerate,  and  in  sand- 
stones. It  is  also  present  in  small  quantities  in  many  volcanic  rocks, 
and  is  disseminated  through  pyrite  (FeS2)  and  some  other  sulphur  com- 
pounds and  their  oxidation  products. 

The  gold  in  quartz  veins  occurs  as  grains  and  scales  scattered  through 
quartz  irregularly,  often  in  such  small  particles  as  to  be  invisible  to  the 
naked  eye,  or  as  aggregates  of  crystals  in  cavities  in  the  quartz.  Pyrite 
is  nearly  always  associated  with  the  gold.  On  surfaces  exposed  to  the 
weather  the  pyrite  rusts  out  and  stains  the  quartz,  leaving  it  cavernous 
or  cellular. 

Most  of  the  world's  supply  of  gold  has  come  from  placers.  These 
are  accumulations  of  sand  or  gravel  in  the  beds  of  old  river  courses. 
The  sands  of  modern  streams  often  contain  considerable  quantities  of 
gold.  Many  of  the  older  streams  were  much  larger  than  the  modern 
ones  draining  the  same  regions  and,  consequently,  their  beds  contain 
more  gold.    This  was  originally  brought  down  from  the  mountains  or 


60  DESCRIPTIVE  MINERALOGY 

highlands  in  which  the  streams  had  their  sources.  The  sands  and 
gravels  were  rolled  along  the  streams'  bottoms  and  their  greater  portion 
was  swept  away  by  the  currents  into  the  lowlands.  The  gold,  however, 
being  much  heavier  than  the  sands  and  pebble  grains,  merely  rolled 
along  the  bottoms,  dropping  here  and  there  into  depressions  from  which 
it  could  not  be  removed.  As  the  streams  contracted  in  volume  the  gold 
grains  were  covered  by  detritus,  or  perhaps  a  lava  stream  flowing  along 
the  old  river  channel  buried  them.  These  buried  river  channels  with 
their  stores  of  sands,  gravels  and  gold  constitute  the  placers.  With  the 
gold  are  often  associated  zircon  crystals,  garnets,  diamonds,  topazes 
and  other  gem  minerals.  Alluvial  gold  is  usually  in  flattened  scales  or 
in  aggregates  of  scales  forming  nuggets.  Some  of  the  nuggets  are  so 
large,  190  pounds  or  more  in  weight,  that  it  is  thought  they  may  have 
been  formed  by  some  process  of  cementation  after  they  were  transported 
to  their  present  positions. 

The  gold-quartz  veins  are  usually  closely  associated  with  igneous 
rocks,  but  the  veins  themselves  may  cut  through  sedimentary  beds  or 
crystalline  schists.  The  veins  are  supposed  to  have  been  filled  from 
below  by  ascending  solutions.  Metallic  gold  is  also  present  in  the  oxi- 
dized zones  of  many  veins  of  gold-bearing  sulphides  and  in  the  zones  of 
secondary  enrichment.  At  the  surface  the  iron  sulphides  are  oxidized 
into  sulphates,  leaving  part  of  the  gold  in  the  metallic  state  and  dissolv- 
ing another  part  which  is  carried  downward  and  precipitated. 

Principal  Localities, — Vein  gold  occurs  in  greater  or  less  quantity  in 
all  districts  of  crystalline  rocks.  It  has  been  obtained  in  large  quantity 
along  the  eastern  flanks  of  the  Ural  Mountains,  this  having  been  the 
most  productive  region  in  the  world  between  the  years  1819  and  1849. 
It  has  been  obtained  also  from  the  Altai  Mountains  in  Siberia,  from  the 
mountains  in  southeastern  Brazil,  from  the  highlands  of  many  of  the 
Central  and  South  American  countries,  and  from  the  western  portion  of 
the  United  States,  more  particularly  from  the  western  slopes  of  the  Sierra 
Nevada  Mountains  and  the  higher  portions  of  the  Rocky  Mountains. 
In  recent  years  auriferous  quartz  veins  have  been  worked  at  various 
points  in  Alaska,  at  Porcupine,  Ontario,  and  other  points  in  Canada. 

The  great  placer  mines  of  the  world  are  in  California,  Australia  and 
Alaska.  In  Australia  the  principal  gold  mines  are  situated  in  the  streams 
rising  in  the  mountains  of  New  South  Wales  and  their  extension  into 
Victoria.  The  valleys  of  the  Yukon  and  other  rivers  in  Alaska  have 
lately  attracted  much  attention,  and  in  the  past  few  years  the  beach 
sands  off  Nome  have  yielded  much  of  the  metal. 

The  most  important  production  at  present  is  from  South  Africa 


INTRODUCTION— THE  ELEMENTS  61 

where  the  metal  occurs  in  an  old  conglomerate.  In  the  opinion  of  some 
geologists  this  is  an  old  beach  deposit;  in  the  opinion  of  others  the  gold 
was  introduced  into  the  conglomerate  long  after  it  had  consolidated. 

The  sands  of  many  streams  in  Europe  and  in  the  eastern  United 
States  have  for  many  years  been  "panned"  or  washed  for  gold.  The 
South  Atlantic  States,  before  the  discovery  of  gold  in  California,  in 
1849,  yielded  annually  about  a  million  dollars'  worth  of  the  precious 
metal.  All  of  it  was  obtained  by  working  the  gravels  and  sands  of  small 
rivers  and  rivulets.  Many  of  these  streams  have  been  worked  over 
several  times  at  a  profit  and  the  mining  continues  to  the  present  day. 
Small  quantities  of  gold  have  also  been  obtained  from  streams  in  Maine, 
New  Hampshire,  Maryland  and  other  Atlantic  coast  states. 

Extraction  and  Refining. — Gold  is  extracted  from  alluvial  sands 
and  from  placers  by  washing  in  pans  or  troughs.  The  sand,  gravel 
and  foreign  particles  are  carried  away  by  currents  of  water  and 
the  gold  settles  down  with  other  heavy  minerals  to  the  bottom  of  the 
shallow  pans  used  in  hand  washing,  or  into  compartments  prepared  for 
it  in  troughs  when  the  processes  are  on  a  larger  scale.  It  is  after- 
ward collected  by  shaking  it  with  mercury  or  quicksilver,  in  which  it 
dissolves.  The  quicksilver  is  finally  driven  off  by  heat  and  the  gold 
left  behind.  Auriferous  beach  sands  and  many  lake,  swamp  and  river 
sands  are  dredged  and  the  sand  thus  raised  is  treated  by  similar  methods. 
Sands  containing  as  low  as  15  cents'  worth  of  metal  per  cubic  yard  can 
be  worked  profitably  under  favorable  conditions. 

Where  the  gold  occurs  free  (not  disseminated  through  sulphides) 
in  quartz  the  rock  is  crushed  to  a  fine  pulp  with  water  and  the  mixture 
allowed  to  flow  over  copper  plates  coated  with  quicksilver.  The  gold 
unites  with  the  quicksilver  and  forms  an  alloy  from  which  the  mercury 
is  driven  off  by  heat.  The  process  of  forming  alloys  of  silver  or  gold 
with  mercury  is  known  as  amalgamation. 

When  the  gold  is  disseminated  through  sulphides,  these  are  concen- 
trated, i.e.,  freed  from  the  gangue  material  by  washing,  and  then 
roasted.  This  liberates  the  gold  which  is  collected  by  amalgamation, 
or  is  dissolved  by  chlorine  or  cyanide  solutions  and  then  precipitated. 

Uses. — Gold,  like  silver,  is  used  in  the  manufacture  of  jewelry  and  or- 
naments, in  the  manufacture  of  gold  leaf  for  gilding  and  in  the  produc- 
tion of  valuable  pigments  such  as  the  "purple  of  Cassius."  It  also  con- 
stitutes the  principle  medium  for  coinage  in  nearly  all  of  the  most 
important  countries  of  the  world.  The  gold  coins  of  the  United  States 
contain  900  parts  gold  in  1,000.  Those  of  Great  Britain  contain  916.66 
parts,  the  remaining  parts  consisting  of  copper  and  silver.    The  total 


62  DESCRIPTIVE  MINERALOGY 

gold  coinage  of  the  United  States  mints  from  the  time  of  their  organi- 
zation to  the  end  of  the  year  191 2  amounted  to  $2,765,900,000.  The 
gold  coined  in  the  world's  mints  in  191 2  amounted  in  value  to  $360,- 
671,382,  and  that  consumed  in  arts  and  industries  to  $174,100,000. 
Jewelers  estimate  the  fineness  of  gold  in  carats,  24-carat  gold  being  pure. 
Eigh teen-carat  gold  is  gold  containing  18  parts  of  pure  gold  and  6  parts 
of  some  less  valuable  metal,  usually  copper.  The  copper  is  added  to 
increase  the  hardness  of  the  metal  and  to  give  it  a  darker  color.  The 
gold  used  most  in  jewelry  is  14  or  1 2  carats  fine. 

Production. — The  total  value  of  the  gold  product  of  the  United 
States  during  191 2  was  $93,451,000.  Of  this  the  following  states  and 
territories  were  the  largest  producers: 

Alaska $17,198,000        Nevada $13,576,000 

California 20,008,000        South  Dakota 7,823,000 

Colorado 18,741,000        Utah 4,312,000 

Of  the  total  product,  placers  yielded  gold  valued  at  $23,019,633,  and 
quartz  veins,  metal  valued  at  $62,112,000.  The  balance  of  the  gold  was 
obtained  from  ores  mined  mainly  for  other  metals,  and  in  these  it  is 
probably  not  in  the  metallic  state.  Moreover,  some  of  the  ore  in  quartz 
veins  is  a  gold  telluride,  but  by  far  the  greater  portion  of  the  product 
from  the  quartz  veins  and  placers  was  furnished  by  the  native  metal. 

The  world's  yield  of  the  precious  metal  in  191 2  was  valued  at  $466,- 
136,100.  The  principal  producing  countries  and  the  value  of  the  gold 
produced  by  each  were : 

South  Africa $211,850,600  Mexico $24,450,000 

United  States 93>45i,5QO  India 11,055,700 

Australasia 54,509,400  Canada 12,648,800 

Russia 22,199,000  Japan 4,467,000 

Lead  occurs  very  rarely  as  octahedral  or  dodecahedral  crystals, 
in  thin  plates  and  as  small  nodular  masses  in  districts  containing  man- 
ganese and  lead  ores  and  also  in  a  few  placers.  It  usually  contains 
small  quantities  of  silver  and  antimony.  The  native  metal  has  the 
same  properties  as  the  commercial  metal.  Its  hardness  is  1.5  and 
density  11.3.     It  melts  at  about  33. 5  °. 

The  mineral  is  of  no  commercial  importance.  The  metal  is  obtained 
from  galena  and  other  lead  compounds. 

Mercury  occurs  as  small  liquid  globules  in  veins  of  cinnabar  (HgS) 
from  which  it  has  probably  been  reduced  by  organic  substances,  and  ig 


INTRODUCTION— THE  ELEMENTS  63 

the  rocks  traversed  by  these  veins.  The  native  metal  possesses  the 
same  properties  as  the  commercial  metal.  It  solidifies  at  —  39  °,  when 
it  crystallizes  in  octahedrons  having  a  cubic  cleavage.  Its  density  is 
13.6.    Its  boiling-point  is  3500. 

The  commercial  metal  is  obtained  from  cinnabar  (p.  98). 

Amalgam  (Ag  •  Hg)  is  found  in  dodecahedral  crystals  in  a  few  places, 
associated  with  mercury  and  silver  ores.  It  occurs  also  as  embedded 
grains,  in  dense  masses  and  as  coatings  on  other  minerals.  It  is  silver- 
white  and  opaque  and  gives  a  distinct  silver  streak  when  rubbed  on 
copper.  Its  hardness  is  about  3  and  its  density  13.9.  When  heated 
in  the  closed  tube  it  yields  a  sublimate  of  mercury  and  a  residue  of 
silver.  On  charcoal  the  mercury  volatilizes,  leaving  a  silver  globule, 
soluble  in  nitric  acid. 

PLATINUM-IRON  GROUP 

The  platinum-iron  group  of  minerals  may  be  divided  into  the  plati- 
num and  the  iron  subgroups.  The  latter  comprises  only  iron  and  nickel- 
irony  both  of  which  are  extremely  rare;  and  the  former,  the  metals 
platinum,  iridium,  osmium,  ruthenium,  rhodium,  and  palladium.  The 
platinum  metals  probably  constitute  an  isodimorphous  group  since 
they  occur  together  in  alloys,  some  of  which  are  isometric  and  others 
hexagonal  (rhombohedral).  Platinum  is  the  only  member  of  the  group 
of  economic  importance. 

Platinum  (Pt) 

Platinum  occurs  but  rarely  in  crystals.  It  is  almost  universally 
found  as  granular  plates  associated  with  gold  in  the  sands  of  streams 
and  rivers,  and  rarely  as  tiny  grains  or  flakes  in  certain  very  basic 
igneous  rocks. 

As  found  in  nature  the  metal  always  contains  iron,  iridium,  rhodium, 
palladium  and  often  other  metals.    A  specimen  from  California  yielded: 

Cu       Ir.Os    Sand     Total 
1.40      1. 10     2.95     101.15 

Though  the  metal  occurs  usually  in  grains  and  plates,  nevertheless 
its  crystals  are  sometimes  found.  On  them  cubic  faces  are  the  most 
prominent  ones,  though  the  octahedrons,  the  dodecahedrons  and 
tetrahexahedrons  have  also  been  identified.  Like  the  crystals  of  silver 
and  gold,  those  of  platinum  are  frequently  distorted. 


Pt 

Au 

Fe 

It 

Rh 

Pd 

85.50 

.80 

6-75 

1  05 

1. 00 

.60 

64  DESCRIPTIVE  MINERALOGY 

The  color  of  platinum  is  a  little  more  gray  than  that  of  silver.  Its 
streak  is  also  gray.  Its  hardness  is  4-4.5  an^  density  14  to  19.  Pure 
platinum  has  a  density  of  21.5.  It  is  malleable  and  ductile,  a  good 
conductor" of  electricity,  and  it  is  infusible  before  the  blowpipe  except 
in  very  fine  wire.  It  is  not  dissolved  by  any  single  acid,  though  soluble, 
like  gold,  in  aqua  regia.     Its  melting  temperature  is  17550. 

Syntheses. — Crystals  have  been  obtained  by  cooling  siliceous  mag- 
mas containing  the  metal,  and  by  dissolving  the  metal  in  saltpeter  and 
cooling  the  mixture. 

Occurrence. — Platinum  is  found  in  the  sands  of  rivers  or  beaches 
and  in  placer  deposits  in  which  it  occurs  in  flattened  scales  or  in 
small  grains.  Nuggets  of  considerable  size  are  sometimes  met  with, 
the  largest  known  weighing  about  i8f  kilos.  It  is  present  also  in 
small  quantity  in  certain  very  basic  igneous  rocks,  like  peridotite. 

Localities. — It  occurs  in  nearly  all  auriferous  placer  districts  and 
in  small  quantities  in  the  sands  of  many  rivers,  among  them  the  Ivalo 
in  Lapland,  the  Rhine,  the  rivers  of  British  Columbia,  and  of  the  Pacific 
States.  It  is  more  abundant  in  the  Natoos  Mountains  in  Borneo,  on 
the  east  flanks  of  the  Ural  Mountains  in  Siberia,  in  the  placer  of  an 
old  river  in  New  South  Wales,  Australia,  and  the  sands  of  rivers  of 
the  Pacific  side  of  Colombia.  It  is  nearly  always  associated  with 
chromite  (p.  200).  A  recent  discovery  which  may  prove  to  be  of  con- 
siderable importance  is  near  Goodsprings,  Nev.,  where  platinum  is  in 
the  free  state  associated  with  gold  in  a  siliceous  ore. 

The  native  metal  is  probably  an  original  constituent  of  some  peri- 
dotites  (basic  igneous  rocks).  Its  presence  in  placers  is  due  to  the 
disintegration  of  these  rocks  by  atmospheric  agencies. 

Extraction  and  Refining. — The  metal  is  separated  from  the  sand 
with  which  it  is  mixed  by  washing  and  hand  picking.  The  metallic 
powder  is  then  refined  by  chemical  methods. 

Uses. — On  account  of  its  infusibility  and  its  power  to  resist  the  cor- 
rosion of  most  chemicals  the  metal  is  used  extensively  for  crucibles 
and  other  apparatus  necessary  to  the  work  of  the  chemist.  It  is  also 
used  by  dentists  and  by  the  manufacturers  of  incandescent  electric 
lamps.  It  is  an  important  metal  in  the  manufacture  of  physical  and 
certain  surgical  instruments,  and  was  formerly  used  by  Russia  for  coin- 
age. The  most  important  use  of  the  metal  in  the  industries  is  in  the 
manufacture  of  sulphuric  acid.  Sulphur  dioxide  (SO2)  and  steam  when 
mixed  and  passed  over  the  finely  divided  metal  unite  and  form  H2SO4. 
:  More  than  half  of  the  acid  made  at  present  is  manufactured  by  this 
:  process. 


INTRODUCTION— THE  ELEMENTS  65 

Production. — Most  of  the  platinum  of  the  world  is  obtained  from 
placers  in  the  Urals  in  Russia.  A  small  quantity  is  washed  from  the 
sands  of  gold  placers  in  Colombia,  Oregon  and  California,  and  an  even 
smaller  quantity  is  obtained  during  the  refining  of  copper  from  the  ores 
of  certain  mines.  The  total  production  of  the  world  in  191 2  was 
314,751  oz.  The  output  for  Russia  in  this  year  was  about  300,000  oz., 
of  Colombia  about  12,000  oz.,  and  of  the  United  States  721  oz.  (equiv- 
alent to  505  oz.  of  the  refined  metal,  valued  at  $22,750).  In  addition, 
about  1,300  oz.  were  obtained  in  the  refining  of  copper  bullion  imported 
from  Sudbury,  Ont.,  and  in  the  treatment  of  concentrates  from  the 
New  Rambler  Mine,  Wyoming.    Of  this  about  500  oz.  were  produced 


Fig.   35. — Iron   Meteorite   (Siderite)   from  Canyon  Diablo,   Arizona.    Weight   365 

lbs.     {Field  Columbian  Museum.) 

from  domestic  ores.    The  importations  into  the  United  States  for  the 
same  year  were  about  125,000  oz.,  valued  at  $4,500,000. 

Platinum-iron,  or  iron-platinum  fPt-Fe),  contains  from  10  per  cent 
to  19  per  cent  Fe.  It  is  usually  dark  gray  or  black  and  is  magnetic.  It 
is  found  with  platinum  in  sands  of  the  rivers  in  the  Urals.  Its  crystals 
are  isometric. 

Iron  (Fe)  occurs  in  small  grains  and  large  masses  in  the  basalt  at 
Ovifak,  Disko  Island,  W.  Greenland,  and  at  a  few  other  points  in  Green- 
land, and  alloys  consisting  mainly  of  iron  are  found  in  the  sands  of  some 
rivers  in  New  Zealand,  Oregon  and  elsewhere.  The  native  metal  always 
contains  some  nickel.  The  most  common  occurrence  of  iron,  however,  is 
in  meteorites  (Fig.  25).     In  these  bodies  also  it  is  alloyed  with  Ni.  When 


66  DESCRIPTIVE  MINERALOGY 

polished  and  treated  with  nitric  acid,  surfaces  of  meteoric  iron  exhibit 

series  of  lines  (Widmanstatten  figures),  that  are  the  edges  of  plates  of 
different  composition  (Fig.  26).  These  are  so  arranged  as  to  indicate 
that  the  substance  crystallizes  in  the  isometric  system. 

Iridium  (Ir-Pt)  and  pUtin-iridium  (Pt-Ir)  are  alloys  of  iridium  and 
platinum  found  as  silver-white  grains  with  a  yellowish  tinge,  associated 
with  platinum  in  the  sands  of  rivers  in  the  Urals,  Burmah  and  Brazil. 
Their  hardness  is  6  to  7,  and  density  22.7.    The  mineral  is  isometric 

and  its  fusing  point  is  between  2iso0-225o". 


FlC.   16. — WidmanslStten  Figures  on   Etched  Surface  of   Meteorite  from  Toluca, 
Mexico.     (One-half  natural  size.)     (Field  Columbian  Museum.) 

Palladium  (Pd)  is  usually  alloyed  with  a  little  Pb  and  Ir.  It  is 
found  in  small  octahedrons  and  cubes  and  also  in  radially  fibrous  grains 
in  the  platinum  sands  of  Brazil,  the  Urals  and  a  few  other  places.  It  is 
whitish  steel-gray  in  color,  has  a  hardness  of  4  to  5  and  a  density  of 
11. 3  to  11. 8.  It  fuses  at  about  1549°.  Its  crystallization  is  isometric. 
About  2,300  oz.  of  the  metal  were  produced  in  the  United  States  during 
1912,  but  all  of  it  was  obtained  during  the  refining  of  bullion.  The 
imports  were  4,967  oz.,  valued  at  $213,397. 

Allopalladium  (Pd)  is  probably  a  dimorph  of  palladium.  It  is  found 
in  six-sided  plates  that  are  probably  rhombohedral,  intimately  asso- 
ciated with  gold,  at  Tilkerode,  Harz. 


INTRODUCTION— THE  ELEMENTS  67 

Osmiridium  (Os  •  Ir)  and  iridosmine  (Ir  •  Os)  are  found  in  crystals  and 
flattened  grains  and  plates  that  are  apparently  rhombohedral.  They 
consist  of  Ir  and  Os  in  different  proportions,  often  with  the  addition 
of  rhodium  and  ruthenium.  Osmiridium  is  tin-white  and  iridosmine 
steel-gray.  Their  hardness  is  6  to  7  and  density  19  to  21.  When  heated 
with  KNO3  and  KOH,  both  yield  the  distinctive  chlorine-like  odor  of 
osmium  oxide  (OSO4)  and  a  green  mass,  which,  when  boiled  with 
water,  leaves  a  residue  of  blue  iridium  oxide.  Both  are  insoluble  in 
concentrated  aqua  regia.  They  occur  with  platinum  in  the  sands  of 
rivers  in  Colombia,  Brazil,  California,  the  Urals,  Borneo,  New  South 
Wales,  and  a  few  other  places.  They  are  distinguished  from  platinum 
by  greater  hardness,  light  color  and  insolubility  in  strong  aqua  regia. 

The  world's  product  of  refined  iridium  is  about  5,000  oz.,  of  which 
the  United  States  furnishes  about  500  oz.  Its  value  is  $63  per  oz. 
Imports  into  the  United  States  during  191 1  were  3,905  oz.,  valued  at 
$210,616.  The  sources  of  the  metal  are  native  iridium,  osmiridium, 
platinum,  copper  ore  and  bullion.  The  metal  is  obtained  from  the  last 
two  sources  in  the  refining  process. 


> 


CHAPTER    IV 

THE  SULPHIDES,  TELLURITES,  SELENIDES,  ARSENIDES  AND 

ANTIMONIDES 

The  sulphides  are  combinations  of  the  metals,  or  of  elements  acting 
like  bases,  with  sulphur.  They  may  all  be  regarded  as  derivatives  of 
hydrogen  sulphide  (H2S)  by  the  replacement  of  the  hydrogen  by  some 
metallic  element.  The  tellurides  are  the  corresponding  compounds  of 
H2Te,  and  the  selenides  of  H2SC 

With  the  same  group  are  also  placed  the  arsenides  and  the  anti- 
monides,  derivatives  of  H3AS  and  HaSb,  because  arsenic  and  antimony 
so  often  replace  in  part  the  sulphur  of  the  sulphides,  forming  with  these 
isomorphous  mixtures. 

The  minerals  described  in  this  volume  may  be  separated  into  the 
following  groups  and  subgroups: 

I.  The  sulphides,  tellurides  and  selenides  of  the  metalloids  arsenic, 
antimony,  bismuth  and  molybdenum. 

II.  The  sulphides,  tellurides,  selenides,  arsenides  and  antimonides 
of  the  metals. 

(a)  The  monosulphides,  etc.     (Derivatives  of  H2S,  EfeSe,  H2Te, 

H3AS,  HsSb.) 

(b)  The  disulphides,  etc.    (Derivatives  of  2H2S,  2H2Te,  2H3AS, 

2HaSb.) 

All  sulphur  compounds  when  mixed  with  dry  sodium  carbonate 
(Na2C03)  and  heated  to  fusion  on  charcoal  yield  a  mass  containing 
sodium  sulphide  (Na2S).  If  the  mass  is  removed  from  the  charcoal, 
placed  on  a  bright  piece  of  silver  and  moistened  with  a  drop  or  two  of 
water  or  hydrochloric  acid,  the  solution  formed  will  stain  the  silver  a 
dark  brown  or  black  color  (Ag2S),  which  will  not  rub  off.  The  sulphides 
yield  the  sulphur  reaction  when  heated  with  the  carbonate  on  platinum 
foil;  the  sulphates  only  when  charcoal  or  some  other  reducing  agent  is 
added  to  the  mixture  before  fusing.  Moreover,  the  sulphides  yield 
sulphureted  hydrogen  when  heated  with  hydrochloric  acid,  while  the 
sulphates  do  not.    These  tests  are  extremely  delicate.     By  the  aid  of 

68 


SULPHIDES,  TELLURIDES,  ETC.  69 

the  first  one  the  sulphur  in  any  compound  may  be  detected.  By  the 
aid  of  the  others  the  sulphates  may  be  distinguished  from  the 
sulphides. 

The  selenides  are  recognized  by  the  strong  odor  evolved  when  heated 
before  the  blowpipe.  Selenates  and  selenites  give  their  odor  only  after 
reduction  with  Na2C03. 

The  tellurides,  when  warmed  with  concentrated  H2SO4,  dissolve  and 
yield  a  carmine  solution  from  which  water  precipitates  a  black  gray 
powder  of  tellurium. 

All  substances  containing  arsenic  and  antimony  yield  dense  white 
fumes  when  heated  on  charcoal  in  the  oxidizing  flame.  The  fumes  of 
arsenic  possess  a  characteristic  odor  while  those  of  antimony  are  odorless. 
When  heated  in  the  open  tube,  arsenides  and  compounds  with  sulphur 
and  arsenic  yield  a  very  volatile  sublimate  composed  of  tiny  white  crys- 
tals (AS2O3).  The  corresponding  sublimate  for  antimonides  and  for 
compounds  with  antimony  and  sulphur  is  nonvolatile,  or  difficultly 
volatile,  and  apparently  amorphous.  It  is  usually  found  on  the  under 
side  of  the  tube. 

THE  SULPHIDES,  SELENIDES  AND  TELLURIDES  OF 

THE  METALLOIDS 

The  sulphides  of  the  metalloids  include  compounds  of  sulphur  with 
arsenic,  antimony,  bismuth  and  molybdenum  and  a  selenide  and  several 
tellurides  of  bismuth.  Only  the  sulphides  are  of  importance.  One, 
stibnite  (Sb2S3),  is  utilized  as  a  source  of  antimony. 

Realgar  (As2S2) 

Realgar  occurs  as  a  bright  red  incrustation  on  other  substances, 
as  compact  and  granular  masses  and  as  crystals  implanted  on  other 
minerals.    It  is  usually  associated  with  the  bright  yellow  orpiment 

(p.  7i)- 

Absolutely  pure  realgar  should   have  the  following  composition: 

As,   70.1   per  cent,  S,  29.9  per  cent.    The  mineral,  however,  usually 

contains  a  small  amount  of  impurities.    It  may  be  looked  upon  as  a 

derivative  of  H2S  in  which  the  hydrogen  of  two  molecules  is  replaced 

by  two  arsenic  atoms,  thus: 

H2S  As=S 

yielding   | 
H2S  As=S. 


70 


DESCRIPTIVE  MINERALOGY 


(c);  PSb,  on  (q)  and  P, 
In  ($). 


Crystals  of  realgar  are  usually  short  and  prismatic  in  habit.  They 
are  monoclinic  (prismatic  class)  with  an  axial  ratio  a  :  b  :  c  :  =1.44". 
1  :  .973  and  0=66°  5'.  The  characteristic  prismatic  faces  are 
(w)ooP(no)  and  (/)  00  P2(2io).  These  with  (b)  00  P  So  (010)  con- 
stitute the  prismatic  zone.  The  terminations  are  (r)£Pob(oi2)  or 
(q)  Poo  (on)  in  combination  with  the  basal  plane  (c)  oP(ooi),  the 
orthodome  (x)  (Toi),  and  one  or  more  of  several  pyramids.    (See  Fig. 

27.)  The  crystals  are  usually  small  and  are 
striated  vertically.  Prismatic  angle  no  A 1  "io 
=  1050  34'. 

The  mineral  possesses  a  distinct  cleavage 
parallel    to   (b)  00  P  00    and  (/)  00  Pi.     It  is 
sectile,  soft  (H=  1.5-2),  resinous  in  luster  and 
aurora-red  or  orange  in  color.    Its  streak  is  a 
lighter  shade,  but  with  the  mineral  are  fre- 
quently intermingled  small  quantities  of  orpi- 
Fig.   27.— Realgar  Crystal,    ment  which  impart  to  its  streak  a  distinct 
ooP,Mo(m);«P2?2io(/);    yellow  tinge.    Its  density  is  3.56.    In  thin 
oop 5b,  010  («;  oP,  001    splinters   it   is  often    translucent    or    trans- 
parent, and  strongly  pleochroic   in  red  and 
yellow  tints,  but  in  masses  it  is  opaque.    Its 
indices  of  refraction  are  not  known  with  accuracy,  but  its  double  re- 
fraction is  strong  (.030).     It  is  a  nonconductor  of  electricity. 

When  heated  on  charcoal  before  the  blowpipe  realgar  catches  fire 
and  burns  with  a  light  blue  flame,  at  the  same  time  giving  off  dense 
clouds  of  arsenic  fumes  and  the  odor  of  burning  sulphur  (SO2).  When 
heated  in  a  closed  tube  it  melts,  volatilizes  and  yields  a  transparent 
red  sublimate  in  the  cold  parts  of  the  tube. 

Its  bright  red  color  and  its  reaction  for  sulphur  distinguish  realgar 
from  all  other  minerals  but  cinnabar,  the  sulphide  of  mercury  (p.  98). 
It  may  easily  be  distinguished  from  cinnabar  by  its  softness,  its  low 
specific  gravity  and  the  arsenic  fumes  which  it  yields  when  heated  on 
charcoal. 

On  exposure  to  the  air  and  to  light  realgar  oxidizes,  yielding  orpi- 
ment  (AS2S3)  and  arsenolite  (AS2O3). 

Syntheses. — Realgar  is  often  produced  in  the  flues  of  furnaces  in 
which  ores  containing  sulphur  and  arsenic  are  roasted.  Crystals  have 
also  been  produced  by  heating  to  1500  a  mixture  of  AsS  with  an  excess 
of  sulphur  in  a  solution  of  bicarbonate  of  soda  sealed  in  a  glass  tube. 

Occurrence  Localities   and  Origin. — Realgar  occurs  in  masses  asso 
ciated  with  orpiment  and  in  grains  scattered  through  it  at  all  places 


SULPHIDES,  TELLURIDES,  ETC.  71 

where  the  latter  mineral  is  found.  It  also  occurs  associated  with  silver 
and  lead  ores  in  many  places.  It  is  found  in  crystals  implanted  on 
quartz  and  on  the  walls  of  cavities  in  lavas.  It  is  also  occasionally 
a  deposit  from  hot  springs.  In  the  United  States  it  forms  seams  in  a 
sandy  clay  in  Iron  Co.,  Utah.  Its  crystals  are  found  in  calcite  in  San 
Bernardino  and  Trinity  Counties,  California,  and  with  orpiment  it  is 
deposited  as  a  powder  by  the  hot  water  of  the  Norris  Geyser  basin  in  the 
Yellowstone  National  Park. 

In  most  cases  it  is  a  product  of  the  interaction  of  arsenic  and  sul- 
phur vapors. 

Uses. — The  native  realgar  occurs  in  too  small  a  quantity  to  be  of 
commercial  importance.  An  artificial  realgar  is  employed  in  tanning 
and  in  the  manufacture  of  "  white-fire." 

Orpiment  (As2S3) 

Orpiment,  though  more  abundant  than  realgar,  is  not  a  common 
mineral.  It  is  usually  found  in  foliated  or  columnar  masses  with  a 
bright  yellow  color.  Its  name — a  contraction  from  the  Latin  auri- 
pigmentum,  meaning  golden  paint — refers  to  this  color. 

The  pure  mineral  contains  39  per  cent  of  sulphur  and  61  per  cent 
of  arsenic,  corresponding  to  the  formula  AS2S3.  It  thus  contains 
about  9  per  cent  more  sulphur  than  does  realgar. 

The  monoclinic  orpiment  crystals  have  the  symmetry  of  the  pris- 
matic class.  Their  axial  ratio  is  .596  :  1  :  .665  with  £=89°  19'.  Though 
always  small  they  are  distinctly  prismatic  with  an  orthorhombic  habit. 
Their  predominant  faces  are  the  ortho  and  clino  pinacoids,  several 
prisms  and  the  orthodome. 

The  cleavage  of  orpiment  is  so  perfect  parallel  to  00  P  00  (010)  that 
even  from  large  masses  of  the  mineral  distinct  foliae  may  be  split. 
These  are  flexible  but  not  elastic.  The  mineral,  like  many  other 
flexible  minerals,  is  sectile.  Its  luster  is  pearly  on  cleavage  faces, 
which  are  always  vertically  striated,  and  is  resinous  on  other  surfaces. 
The  color  of  pure  orpiment  is  lemon-yellow;  it  shades  into  orange 
when  the  mineral  is  impure  through  the  admixture  of  realgar.  Its 
streak  is  always  of  some  lighter  shade  than  that  of  the  mineral.  Its 
hardness  is  1.5-2  and  its  density  about  3.4.  In  small  pieces  orpiment 
is  translucent  and  possesses  an  orange  and  greenish  yellow  pleochroism. 
When  heated  to  ioo°  it  becomes  red  and  assumes  the  pleochroism  of 
realgar.  It,  however,  resumes  its  characteristic  color  and  pleochroism 
upon  cooling.  When  heated  to  1500  the  change  is  permanent.  The 
mineral  is  a  nonconductor  of  electricity. 


72 


DESCRIPTIVE  MINERALOGY 


The  chemical  properties  of  orpiment  are  the  same  as  those  described 
for  realgar,  except  that  the  sublimate  in  the  closed  tube  is  yellow  instead 
of  red. 

Synthesis. — Orpiment  is  produced  in  large  pleochroic  crystals  by 
treatment  of  arsenic  acid  with  H2S  under  high  pressure. 

Occurrence,  Localities  and  Origin. — Orpiment  occurs  in  the  same 
forms  and  in  the  same  places  as  does  realgar.  Small  specks  of  it  occur 
on  arsenical  iron  at  Edenville,  N.  Y.  It  is  also  found  in  the  deposits 
of  Steamboat  Springs,  Nevada.  The  origin  of  orpiment  is  similar 
to  that  of  realgar.     It  is  also  formed  by  the  oxidation  of  this  mineral. 

Uses. — Native  orpiment  mixed  with  water  and  slaked  lime  is  used 
in  the  East  as  a  wash  for  removing  hair.  It  is  also  employed  as  a  pig- 
ment in  dyeing.  Most  of  the  AS2S3  of  commerce  is  a  manufactured 
product. 

STIBNITE  GROUP  (RaQ8) 

The  stibnite  group  of  sulphides  contains  several  isomorphous 
compounds,  of  which  we  shall  consider  only  two,  viz.,  stibnite  (Sb2Ss) 
and  bismuthinite  (Bi2S3).  The  general  formula  of  the  group  is  R2Q3, 
in  which  R  stands  for  Sb  or  Bi  and  Q  for  S  or  Se.  The  group  is 
orthorhombic  (bipyramidal  class).  All  the  members  have  a  distinct 
cleavage  parallel  to  the  brachypinacoid  which  yields  flexible  laminae. 


m 


m 


Stibnite  (Sb2S3) 

Stibnite  is  the  commonest  and  the  most  important  ore  of  anti- 
mony. It  is  found  in  acicular  and  prismatic  crys- 
tals, in  radiating  groups  of  crystals  and  in 
fibrous  masses. 

Chemically,  stibnite  is  the  antimony  trisul- 
phide,  Sb2S3,  composed  of  Sb,  71.4  per  cent 
and  S,  28.6  per  cent.  As  found,  however,  it 
usually  contains  small  quantities  of  iron  and  often 
traces  of  silver  and  gold. 

Crystals  of   stibnite   are  often  very  compli- 
cated.   They  are  orthorhombic  with  an  axial  ratio 
.9926  :  1  :  1. 01 79   and   a   columnar  or    acicular 
habit.    The  most  important  forms  in  the  pris- 
matic zone  are  00  P(no)  and  00  P  06  (010).    The 
prisms  are  often  acutely  terminated  by  P(in),  ^4(431)  and  6P2(36i), 
or  bluntly  terminated  by  £P(ii3),  (Fig.  28).    Sometimes  the  crystals 
are  rendered  very  complicated  by  the  great  number  of  their  terminal 


Fig.  28.— Stibnite  Crys- 
tal. 00  P,  110  (w); 
00  Poo  ,  010  (6);  2P2, 
121  (t>)  andP,  in  (p). 


SULPHIDES,  TELLURIDES,  ETC.  73 

planes.  Dana  figures  a  crystal  from  Japan  that  possesses  a  termina- 
tion of  84  planes,     no  a  1 10=89°  34'. 

Many  of  the  crystals  of  this  mineral,  more  particularly  those  with 
an  acicular  habit,  are  curved,  bent  or  twisted.  Nearly  all,  whether 
curved  or  straight,  are  longitudinally  striated. 

The  cleavage  of  stibnite  is  very  perfect  parallel  to  00  P  06  (010), 
leaving  striated  surfaces.  The  mineral  is  soft  (H=2)  and  slightly 
sectile.  Its  density  is  about  4.5.  Its  color  is  lead-gray  and  its  streak 
a  little  darker.  In  very  thin  splinters  it  is  translucent  in  red  or  yellow 
tints.  In  these  the  indices  of  refraction  for  yellow  light  have  been 
determined  to  be,  0=4.303  and  7  =  3.194.  Surfaces  that  are  exposed 
to  the  air  are  often  coated  with  a  black  or  an  iridescent  tarnish.  The 
luster  of  the  mineral  is  metallic.     It  is  a  nonconductor  of  electricity. 

Stibnite  fuses  very  easily,  thin  splinters  being  melted  even  in  the 
flame  of  a  candle.  When  heated  on  charcoal  the  mineral  yields  anti- 
mony and  sulphurous  fumes,  the  former  of  which  coat  the  charcoal  white 
in  the  vicinity  of  the  assay.  When  heated  in  the  open  tube  SO2  is 
evolved  and  a  white  sublimate  of  SD2O3  is  deposited  on  the  cool  walls  of 
the  tube.  In  the  closed  tube  the  mineral  gives  a  faint  ring  of  sulphur 
and  a  red  coating  of  antimony  oxysulphide.  It  is  soluble  in  nitric  acid 
with  the  precipitation  of  SD2O5. 

Stibnite  may  easily  be  distinguished  from  all  minerals  but  the  other 
sulphides  by  the  test  for  sulphur.  From  the  other  sulphides  it  is  dis- 
tinguished by  its  cleavage  and  the  fumes  it  yields  when  heated  on  char- 
coal. Its  closest  resemblance  is  with  galena  (PbS),  which,  however, 
differs  from  it  in  being  less  fusible  and  in  yielding  a  lead  globule  when 
fused  with  sodium  carbonate  on  charcoal.  Moreover,  galena  possesses 
a  cubic  cleavage. 

Syntheses. — Stibnite  is  produced  by  heating  to  2000,  a  mixture  of 
sulphur  and  antimony  with  water  under  pressure,  and  by  the  reaction  of 
H2S  on  antimony  oxide  heated  to  redness. 

Occurrence,  Localities  and  Origin.— -The  mineral  is  found  as  crystals 
in  quartz  veins  cutting  crystalline  rocks,  and  in  metalliferous  veins  asso- 
ciated with  lead  and  zinc  ores,  with  cinnabar  (HgS)  and  barite  (BaS04). 
The  finest  crystals,  some  of  them  20  inches  in  length,  come  from  mines 
in  the  Province  of  Iyo,  on  the  Island  of  Shikoku,  Japan.  The  mineral 
occurs  also  in  York  Co.,  New  Brunswick,  in  Rawdon  township,  Nova 
Scotia,  at  many  points  in  the  eastern  United  States,  in  Sevier  Co., 
Arkansas,  in  Garfield  Co.,  Utah,  and  at  many  of  the  mining  districts  in 
the  Rocky  Mountain  States. 

In  Arkansas  stibnite  is  in  quartz  veins  following  the  bedding  planes 


74  DESCRIPTIVE  MINERALOGY 

of  shales  and  sandstones.  With  it  are  found  many  lead,  zinc  and 
iron  compounds  and  small  quantities  of  rarer  substances.  In  Utah 
the  mineral  occurs  in  veins  unmixed  with  other  minerals,  except  its 
own  oxidation  products.  The  veins  follow  the  bedding  of  sandstones 
and  conglomerates.  Here,  as  in  Arkansas,  the  stibnite  is  believed  to 
have  been  deposited  by  magmatic  waters. 

Uses. — Stibnite  was  powdered  by  the  ancients  and  used  to  color  the 
eyebrows,  eyelashes  and  hair.  At  present  it  is  used  to  a  slight  extent  in 
vulcanizing  rubber  and  in  the  manufacture  of  safety  matches,  percussion 
caps,  certain  kinds  of  fireworks,  etc.  Its  principal  value  is  as  an  ore  of 
antimony.  Practically  all  of  the  metal  used  in  the  arts  is  obtained 
from  this  source.  Antimony  is  chiefly  valuable  as  an  alloy  with  other 
metals.  With  tin  and  lead  it  forms  type  metal.  The  principal  alloys 
with  tin  are  britannia  metal  and  pewter.  With  lead,  tin  and  copper 
it  constitutes  babbit  metal,  a  hard  alloy  used  in  the  construction  of 
locomotive  and  car  journals,  and  with  other  substances  it  enters  into 
the  composition  of  other  alloys  used  for  a  variety  of  purposes.  The 
double  tartrate  of  antimony  and  potassium  is  the  well  known  tartar 
emetic.    The  pigment,  Naples  yellow,  is  an  antimony  chromate. 

Production. — The  total  quantity  of  stibnite  mined  in  the  world  can- 
not be  accurately  estimated.  That  mined  in  the  United  States  is  very 
small  in  amount,  most  of  the  antimony  produced  in  this  country  being 
obtained  in  the  form  of  an  antimony  alloy  as  a  by-product  in  the  smelting 
of  antimonial  lead  ores. 

Blsmuthinite  (Bi2S3) 

Bismuthinite  is  completely  isomorphous  with  stibnite.  It  rarely, 
however,  occurs  in  acicular  crystals,  but  is  more  frequently  in  foliated, 
fibrous  or  dense  masses. 

Its  axial  ratio  is  .968  :  1  :  .985. 

The  angle  1 10  A  iTo  =  &&°  8'. 

The  mineral  resembles  stibnite  in  color  and  streak,  but  its  surface  is 
often  covered  with  a  yellowish  iridescent  tarnish.  Its  fusibility  and 
hardness  are  the  same  as  those  of  stibnite  but  its  density  is  6.8-7.1.  It 
is  an  electrical  conductor. 

In  the  open  tube  the  mineral  yields  SO2  and  a  white  sublimate 
which  melts  into  drops  that  are  brown  while  hot,  but  change  to  opaque 
yellow  when  cold.  On  charcoal  it  yields  a  coating  of  yellow  Bi203  which 
changes  to  a  bright  red  BH3  when  moistened  with  potassium  iodide. 
The  mineral  dissolves  in  hot  nitric  acid,  forming  a  solution,  which  upon 
the  addition  of  water  gives  a  white  precipitate  of  a  basic  bismuth  nitrate. 


SULPHIDES,  TELLURIDES,  ETC.  75 

Bismuthinite  is  distinguished  from  stibnite  by  the  coating  on  char- 
coal and  by  its  complete  solubility  in  HNO3. 

Syntheses. — Crystals  have  been  obtained  by  cooling  a  solution  of 
Bi2Ss  in  molten  bismuth,  and  by  cooling  a  solu  Jon  made  by  heating 
IM2S3  in  a  solution  of  potassium  sulphide  in  a  closed  tube  at  2000. 

Occurrence,  Localities  and  Origin. — Bismuthinite  occurs  as  a  constit- 
uent of  veins  associated  with  quartz,  bismuth  and  chalcopyrite,  in  which 
it  was  probably  formed  as  a  product  of  pneumatolytic  processes.  It  is 
found  at  Schneeberg  and  other  points  in  Saxony;  at  Redruth  and 
elsewhere  in  Cornwall;  near  Beaver  City,  Utah;  in  a  gold-bearing  vein 
at  Gold  Hill,  Rowan  County,  N.  C;  and  in  a  vein  containing  beryl, 
garnet,  etc.,  in  granite  at  Haddam,  Conn. 

TETRADYMITE    GROUP 

This  group  comprises  a  series  of  tellurides  and  selenides  of  bismuth 
that  have  not  been  satisfactorily  differentiated  because  of  the  lack  of 
accurate  analyses. 

Tetradymite,  the  best  known  member  of  the  group,  is  probably  an 
isomorphous  mixture  cf  bismuth  telluride  and  bismuth  sulphide  of  the 
formula  Bi2(Te-S)3.  It  occurs  in  small  rhombohedral  crystals  with  the 
axial  ratio  1  :  1.587  and  10T1 A  1101  =  98°  58'.  Its  crystals  are  bounded 
by  rhombohedrons  (R(ioTi)  and  2R(202i))  and  the  basal  plane 
(oP(oooi)).  Interpenetration  fourlings  are  common  with  —  5R(oil2), 
the  twinning  plane.  The  mineral  is,  however,  more  frequently  found 
in  foliated  and  granular  masses.  Its  color  is  lead-gray.  It  possesses  a 
perfect  cleavage  parallel  to  the  base.  Its  hardness  is  1.5-2  and  its 
density  about  7.4.  It  is  a  good  electrical  conductor.  Its  best  known 
occurrences  are  Zsubkau,  Hungary,  Whitehall,  Va.,  in  Davidson 
County,  N.  C,  near  Dahlonega,  Ga.,  near  Highland,  Mont.,  and  at 
the  Montgomery  Mine  and  at  Bradshaw  City  in  Arizona.  It  occurs  in 
quartz  veins  associated  with  gold  in  the  gold  sands  of  some  streams. 

The  other  members  of  the  group  appear  to  be  completely  isomorphous 
with  tetradymite.  They  vary  in  color  from  tin-white  through  gray  to 
black. 


Molybdenite  (MoS^   / 

This  mineral,  which  is  the  sulphide  of  the  rare  metal  molybdenum, 
does  not  occur  in  large  quantity,  but  it  is  so  widely  distributed  that  it 
seems  to  be  quite  abundant.     It  occurs  principally  in  black  scales  scat- 


76  DESCRIPTIVE  MINERALOGY 

tered  through  coarse-grained,  crystalline,  siliceous  rocks  and  granular 
limestones  and  in  black  or  lead-gray  foliated  masses. 

The  theoretical  composition  of  molybdenite  is  40  per  cent  sulphur 
and  60  per  cent  molybdenum.  Usually,  however,  the  mineral  contains 
small  quantities  of  iron  and  occasionally  other  components. 

Crystals  of  molybdenite  are  exceedingly  rare.  Scales  and  plates 
with  hexagonal  outlines  are  often  met  with  but  they  do  not  usually  pos- 
sess sufficiently  perfect  faces  to  yield  accurate  measurements.  The 
measurements  that  have  been  obtained  appear  to  indicate  a  holohedral 
hexagonal  symmetry  with  an  axial  ratio  1  :  1.908. 

The  cleavage  of  molybdenite  is  very  perfect  parallel  to  the  base. 
The  laminae  are  flexible  but  not  elastic.  The  mineral  is  sectile  and  so 
soft  that  it  leaves  a  black  mark  when  drawn  across  paper.  Its  density 
is  4.7.  Its  luster  is  metallic,  color  lead-black,  and  streak  greenish 
black.  In  very  thin  flakes  the  mineral  is  translucent  with  a  green  tinge. 
Otherwise  it  is  opaque.  It  is  a  poor  conductor  of  electricity  at  ordi- 
nary temperature,  but  its  conductivity  increases  with  the  temperature. 

In  the  blowpipe  flame  molybdenite  is  infusible.  It,  however,  im- 
parts to  the  edges  of  the  flame  a  yellowish  green  color.  Naturally,  it 
fields  all  the  reactions  for  sulphur,  and  in  the  open  tube  it  deposits  a 
pale  yellow  crystalline  sublimate  of  M0O3.  Molybdenite  is  decomposed 
by  nitric  acid  with  the  production  of  a  gray  powder  (M0O3). 

By  its  color,  luster  and  softness  molybdenite  is  easily  distinguished 
from  all  minerals  but  graphite.  From  this  it  is  distinguished  by  its 
reaction  for  sulphur.  Moreover,  a  characteristic  test  for  all  molyb- 
denum compounds  is  the  dark  blue  coating  produced  on  porcelain  when 
the  pulverized  substance  is  moistened  with  concentrated  sulphuric 
acid  and  then  heated  until  almost  dry.  Before  this  test  can  be  applied 
to  molybdenite,  the  mineral  must  first  be  powdered  and  then  oxi- 
dized by  roasting  in  the  air  for  a  few  minutes  or  by  boiling  to  dryness 
with  a  few  drops  of  HNO3. 

Syntheses. — Crystalline  molybdenite  has  been  prepared  by  the  action 
of  sulphur  vapor  or  H2S  upon  glowing  molybdic  acid.  It  has  also  been 
produced  by  heating  a  mixture  of  molybdates  and  lime,  in  a  large  excess 
of  a  gaseous  mixture  of  HC1  and  H2S. 

Occurrence,  Localities  and  Origin. — Molybdenite  generally  occurs 
embedded  as  grains  in  limestone  and  in  the  crystalline  silicate  rocks, 
as,  for  instance,  granite  and  gneiss,  and  as  masses  in  quartz  veins,  at 
Arendal,  Norway;  at  Blue  Hill  Bay,  Maine;  at  Haddam,  Conn.;  in 
Renfrew  Co.,  Ontario,  and  at  many  points  in  the  far  western  states. 
It  is  thought  to  be  of  pneumatolytic  origin. 


SULPHIDES,  TELLURIDES,  ETC.  77 

Uses. — The  mineral  is  the  principal  ore  of  the  metal  molybdenum, 
the  salts  of  which  are  important  chemicals  employed  principally  in 
analytical  work,  especially  in  the  detection  and  estimation  of  phosphoric 
acid.  The  molybdate  of  ammonia  (NH^MoOi,  the  principal  salt 
employed  in  analytical  processes,  is  easily  obtained  by  roasting  a  mix- 
ture of  sand  and  molybdenite  and  treating  the  oxidized  product  with 
ammonia.  Other  molybdenum  salts  are  used  for  giving  a  green  color 
to  porcelain.  The  metal  is  used  in  an  alloy  (ferro-molybdenum)  for 
hardening  steel,  as  supports  for  the  lower  ends  of  tungsten  filaments  in 
electric  lamps  and  for  making  ribbons  used  in  electric  furnaces. 

Production. — There  was  no  production  of  molybdenite  in  North 
America  during  191 2.  The  imports  of  the  metal  into  the  United  States 
aggregated  3.5  tons,  valued  at  $4,670.  The  value  of  the  imports 
of  the  ore  is  not  known, 

THE  SULPHIDES,  SELENIDES,  ETC.,  OF  THE  METALS  . 
THE  METALLIC  MONO  SULPHIDES,  ETC. 

The  metallic  monosulphides,  monoselenides,  etc.,  are  compounds 
in  which  the  hydrogen  of  H2S,  H2Se,  H2Te,  H3AS,  and  IfcSb  are 
replaced  by  metals.  Among  them  are  some  of  the  most  important 
ores. 

They  may  be  separated  into  several  groups  of  which  some  are 
among  the  best  defined  of  all  the  mineral  groups,  while  others  consist 
simply  of  a  number  of  minerals  placed  together  solely  for  convenience 
of  description.  In  addition,  there  are  a  few  members  of  this  chemical 
group  which  seem  to  have  no  close  relationship  with  any  other  mem- 
bers.   These  are  discussed  separately. 

The  groups  described  are  as  follows: 

The  Dyskrasite  Group. 
The  Galena  Group. 
The  Chalcocite  Group. 
The  Blende  Group. 
The  Millerite  Group. 
The  Cinnabar  Group. 

DYSKRASITE  GROUP 

This  group  includes  a  number  of  arsenides  and  antimonides,  some 
of  which  apparently  contain  an  excess  of  the  metal  above  that  neces- 
sary to  satisfy  the  formulas  H3AS  and  HaSb.    Although  their  com- 


78  DESCRIPTIVE  MINERALOGY 

position  is  not  understood,  they  are  generally  regarded  as  basic  com- 
pounds. A  few'  of  I  hem  are  well  crystallized,  but  their  composition  is 
dcubtful,  because  of  the  difficulty  of  obtaining  pure  material  for  anal- 
yses. Seme  of  them  are  probably  mixtures.  The  members  of  the 
group,  all  of  which  are  cemparatively  rare,  are  whitneyiie  (CugAs), 
algodonite  (CueAs),  domeykite  (CU3AS),  horsjordite  (CuoSb)  and  dyskras- 
ite (Ag3Sb).  Other  minerals  are  known  which  may  properly  be  placed 
here,  but  their  identity  is  doubtful.  The  only  two  members  that  need 
further  discussion  are  domeykite  and  dyskrasite. 

Domeykite  (C113AS)  is  known  only  in  disseminated  particles  and 
in  botryoidal  and  dense  masses  and  small  orthorhombic  crystals.  It 
may  be  a  mixture  of  several  components,  which  in  other  proportions 
form  algodonite.  It  is  tin-white  or  steel-gray  and  opaque.  It  becomes 
dull  and  covered  with  a  yellow  or  brown  iridescent  tarnish  when  ex- 
posed to  the  air.  Its  hardness  is  3-4  and  density  about  7.3.  It  is  the 
most  easily  fusible  of  the  copper  arsenides.  Its  principal  occurrences 
are  in  the  silver  mines  of  Copiap6  and  Coquimbo  in  Chile;  associated 
with  native  copper  at  Cerro  de  Paracabas,  Guerrero,  Mexico;  at  Shel- 
don, Portage  Lake,  Michigan,  and  on  Michipicoten  Island,  in  Lake 
Superior,  Ontario.    The  last  two  occurrences  are  in  quartz  veins. 

Dyskrasite  (Ag3Sb)  occurs  in  foliated,  granular  and  structureless 
masses  and  rarely  in  small  orthorhombic  crystals  with  an  hexagonal 
habit.  Their  axial  ratio  is  .5775  :  1  :  .6718.  Twinning  is  frequent, 
yielding  star-shaped  aggregates.  The  mineral  has  a  silver-white  color 
and  streak,  but  its  exposed  surfaces  are  often  tarnished  yellow  or  black. 
It  is  opaque  and  sectile.  Its  hardness  is  3.5-4  and  density  about  9.6. 
It  is  a  good  electrical  conductor.  Dyskrasite  is  soluble  in  HNO3 
leaving  a  white  sediment  of  SD2O3.  It  occurs  principally  in  the  silver 
mines  of  central  Europe,  and  especially  near  Wolfach,  Baden;  St. 
Andreasberg,  Harz;  and  at  Carrizo,  in  Copiap6,  Chile. 

GALENA  GROUP 

The  minerals  comprising  the  galena  group  number  about  a  dozen 
crystallizing  in  the  holohedral  division  of  the  regular  system  (hex- 
octahedral  class).  They  possess  the  general  formula  RQ  in  which 
R  represents  silver,  lead,  copper  and  gold,  and  Q  sulphur,  selenium 
and  tellurium.  The  group  may  be  divided  into  silver  compounds  and 
lead  compounds,  thus:  (A)  argentite  (Ag2S),  hessite  (Ag2Te),  petzite 
((Ag-Au)2Te),    naumannite    (Ag2Se),    aguilarile    (Ag2(Se-S)),  jalpaite 


SULPHIDES,  TELLURIDES,  ETC.  79 

((Ag-Cu)2S)  and  eukarite  ((Ag-Cu)2Se),  and  (B)  galena  (PbS),  altaite 
(PbTe),  and  clausthalitc  (PbSc).  Of  these  only  two  are  of  importance, 
viz.,  galena,  and  argentite.  Hessite  and  petzite  are  comparatively 
unimportant  ores  of  gold. 

Argentite  (Ag2S) 

Argentite,  though  not  very  widespread  in  its  occurrence,  is  an 
important  ore  of  silver.  It  is  found  in  masses,  as  coatings,  and  in  crys- 
tals or  arborescent  groups  of  crystals. 

Argentite  contains  87.1  per  cent  silver  and  12.9  per  cent  sulphur  when 
pure.  It  is  usually,  however,  impure  through  the  admixture  of  small 
quantities  of  Fe,  Pb,  Cu,  etc. 

The  forms  most  frequently  observed  on  argentite  crystals  are 
00  O  00  (100),  ooO(no)  and  O(in),  though  various  f»Ooo  (hlo)  and 
mOtn  (Ml)  forms  are  also  met  with.  The  crystals  are  often  distorted 
and  often  they  are  grouped  in  parallel  growths  of  different  shapes. 
Twinning  is  common,  with  O(ni)  the  twinning  plane.  The  twins 
are  usually  penetration  twins.  The  habit  of  most  crystals  is  cubical 
or  octahedral. 

Argentite  is  lead-gray  in  color.  Its  streak  is  a  little  darker.  The 
mineral  is  opaque.  Its  luster  is  metallic,  its  hardness  about  2.25  and 
density  7.3.  It  is  sectile,  has  an  imperfect  cleavage  and  is  a  conductor 
of  electricity. 

When  heated  on  charcoal  argentite  swells  and  fuses,  yielding  sulphur 
fumes  and  a  globule  of  silver.     It  is  soluble  in  nitric  acid. 

Argentite  is  easily  recognized  by  its  color,  its  sectility,  the  fact  that 
it  yields  a  silver  globule  when  fused  with  Na2CC>3  on  charcoal  and  yields 
.  the  sulphur  test  with  a  silver  coin. 

Syntheses. — Crystals  of  argentite  may  be  obtained  by  treating  red 
hot  silver  with  sulphur  vapor  or  dry  H2S,  and  by  heating  silver  and  SO2 
in  a  closed  tube  at  2000. 

Occurrence,  Localities  and  Origin. — The  mineral  is  found  in  the  second- 
ary enrichment  zones  of  veins  associated  with  silver  and  other  sulphides 
in  many  silver-mining  districts.  In  Nevada  it  is  an  important  ore  at 
the  Comstock  lode  and  in  the  Cortez  district.  It  is  found  also  near 
Port  Arthur  on  the  north  shore  of  Lake  Superior,  in  Ontario,  and  asso- 
ciated with  native  silver  in  the  copper  mines  of  Michigan.  The  ores  of 
Mexico,  Chile,  Bolivia  and  Peru  are  composed  largely  of  this  mineral. 

Production. — Much  of  the  silver  produced  in  this  country  is  obtained 
from  argentite,  though  by  no  means  so  great  a  quantity  as  is  obtained 
from  other  sources. 


Fe 

Zn 

SiOz 

Total 

i-35 

•      •      • 

■      •      • 

99.96 

•36 

•IS 

.70 

100.01 

1.28 

.21 

.o<? 

100.00 

80  DESCRIPTIVE  MINERALOGY 

Hessite  (Ag2Te)  and  Petzite  ((Ag*Au)2Te) 

These  two  minerals,  though  comparatively  rare,  are  prominent 
sources  of  gold  and  silver  in  some  mining  camps.  They  usually  occur 
together  associated  with  other  sulphides. 

Hessite  is  the  nearly  pure  silver  telluride  and  petzite,  an  isomorphous 
mixture  of  gold  and  silver  tellurides,  as  indicated  by  the  following  analy- 
ses of  materials  from  the  Red  Cloud  Mine,  Boulder  Co.,  Colorado. 

Te  Ag  Au  Cu      Pb 

I.    37.86  59.91  .22  .17      .45 

II.    34.91  50.66  13.09  .07      .17 

III.    32.97  40.80  24.69  

The  minerals  crystallize  in  all  respects  like  argentite.  They  are 
opaque  and  lead-gray  to  iron-black  in  color,  sectile  to  brittle,  have  a 
hardness  between  2  and  3  and  a  specific  gravity  of  8.3-9,  increasing  with 
the  percentage  of  gold  present.    They  are  good  conductors  of  electricity. 

Before  the  blowpipe,  both  minerals  melt  easily  to  a  black  globule,  at 
the  same  time  coloring  the  reducing  flame  greenish  and  giving  the  odor 
of  tellurium  fumes.  When  acted  upon  by  the  reducing  flame,  the  globule 
becomes  covered  with  little  crystals  of  silver.  With  Na2C03  on  charcoal 
both  minerals  yield  a  globule  of  silver,  but  the  globule  obtained  from 
hessite  dissolves  in  warm  HNO3,  while  that  obtained  from  petzite 
becomes  yellow  (gold).  In  the  open  tube  both  yield  a  white  sublimate 
of  TeCfe  which  melts,  when  heated,  to  colorless  drops.  When  heated 
with  concentrated  H2SO4,  they  give  a  purple  or  red  solution  which,  upon 
the  addition  of  water,  loses  its  color  and  precipitates  blackish  gray, 
powdery  tellurium.  The  minerals  dissolve  in  HNO3.  From  this  solu- 
tion HCl  throws  down  white  silver  chloride. 

Both  the  minerals  resemble  very  closely  many  forms  of  argentite 
and  galena,  from  which,  however,  they  may  be  distinguished  by  the 
reactions  for  tellurium.  Petzite  and  hessite  may  be  distinguished  from 
one  another  by  the  test  for  gold.  Moreover  a  fresh  surface  of  hessite 
blackens  when  treated  with  a  solution  of  KCN,  whereas  a  surface  of 
petzite  remains  unaffected. 

Syntheses. — Octahedrons  of  hessite  are  obtained  by  the  action  of 
tellurium  vapor  upon  glowing  silver  in  an  atmosphere  of  nitrogen,  and 
dodecahedrons  of  petzite  upon  similar  treatment  of  gold-silver  alloy. 

Origin. — Both  minerals  are  believed  to  be  primary  deposits  orig- 
inating in  magmatic  solutions.  They  occur  in  veins  with  native  gold, 
quartz,  fluorite,  dolomite,  and  various  sulphides  and  other  tellurides. 


SULPHIDES,  TELLURIDES,  ETC.  81 

Localities. — These  tellurides,  together  with  others  to  be  described 
later  (p.  113),  are  important  sources  of  silver  and  gold  in  the  mines  at 
Nagyag,  Transylvania,  at  Cripple  Creek  and  in  Boulder  Co.,  Colo.,  and 
at  Kalgoorlic,  W.  Australia.  The  quantity  of  tellurides  mined  is  con- 
siderable, but  since  it  is  impracticable  to  separate  these  two  tellurides 
from  the  other  compounds  of  gold  and  silver  mined  with  them,  it  is  im- 
possible to  estimate  the  proportion  of  the  metals  obtained  from  them. 

Galena  (PbS) 

Galena,  the  most  important  ore  of  lead,  occurs  in  great  lead-gray 
crystalline  masses,  in  large  and  small  crystals,  in  coarse  and  fine  granular 
aggregates,  and  in  other  less  common  forms.  Much  galena  contains 
silver,  in  which  case  it  becomes  an  important  ore  of  this  metal. 

Galena  rarely  approaches  the  theoretical  composition  13.4  per  cent 
cf  sulphur  and  86.6  per  cent  of  lead.  It  usually  contains  small  quanti- 
ties of  the  sulphides  cf  silver,  zinc,  cadmium,  copper  and  bismuth  and 
in  some  cases  native  silver  and  gold.  When  the  percentage  of  silver 
present  reaches  3  oz.  per  ton  the  mineral  is  ranked  as  a  silver  ore.  This 
silver  is  apparently  present  in  some  cases  as  an  isomorphous  mixture 
of  silver  sulphide  and  in  other  cases  in  distinct 
minerals  included  within  the  galena. 

"  Galena  crystals  usually  possess  a  cubical  habit, 
though  crystals  with  the  octahedral  habit  are 
very  common.  The  principal  forms  observed  are 
ooOoo(ioo),  O(ni),  ooO(no),  mO<x>(hlo)  and 
mOm  (hll)  (Figs.  29  and  30).  Twins  are  common, 
with  O  the  twinning  face.  FlG-  ^.-Galena  Crys- 

Galena  is  well  characterized  by  its  lead-gray     ,    '        ~      '     /JX 

.  (0);      °°0,    no   (a) 

color,  its  perfect  cleavage  parallel  to  the  cubic  faces     and  O,  in  (0). 
and  by  its  great  density  (8.5).     Its  luster  is  me- 
tallic and  its  hardness  about  2.6.    Its  streak  is  grayish  black.    It  is  a 
good  conductor  of  electricity. 

On  charcoal  galena  fuses,  yielding  sulphurous  fumes  and  a  globule 
of  metallic  lead,  which  may  easily  be  distinguished  from  a  silver  globule 
by  its  softness.  The  charcoal  around  the  assay  is  coated  with  a  yellow 
sublimate  of  lead  oxide  (PbO).  The  mineral  is  soluble  in  HNO3  with 
the  separation  of  sulphur. 

Its  color  and  luster  distinguish  galena  from  nearly  all  minerals  but 
5  Unite.  From  this  mineral  it  is  easily  distinguished  by  its  more  difficult 
fusibility,  by  its  cleavage,  and  by  the  fact  that  it  does  not  yield  the  anti- 
mony fumes  when  heated  on  charcoal. 


82  DESCRIPTIVE  MINERALOGY 

Galena  weathers  readily  to  the  sulphate  (angieske)  and  carbonate 
(cerussite);  consequently  it  is  usually  not  found  in  the  upper  portions 
of  veins  that  are  exposed  to  the  action  of  the  air. 

Syntheses.— Crystals  of  galena  result  from  heating  a  mixture  of 
lead  oxide  with  NH4CI  and  sulphur,  and  from  treatment  of  a  lead  salt 
with  H2S  at  a  red  heat.     Small  crystals  have  been  produced  by  heating 


Fir..  30. — Galena  Crystals  («0™(ioo)  and  O(iti))  partly  covered   by  Martasilc; 
Irom  the  Joplin  Distri-.i,  Mo.     (Aft,r  II'.  S.  T.  Smith  ,ind  C.  E.  Submittal.) 

in  a  sealed  glass  tube  at  8o°-oo°  pulverized  cerussite  (PbCOa)  in  a  water 
solution  of  H2S. 

Origin. — Veins  of  galena  containing  silver  (silver-lead)  were  probably 
produced  by  ascending  solutions  emanating  from  bodies  of  igneous 
rocks,  while  the  galena  in  limestone  was  probably  deposited  by  ground- 
water that  dissolved  the  sulphide  from  the  surrounding  sedimentary 
rocks.     Galena  is  also  in  some  cases  a  melamorphic  product. 

Occurrence. — The  mineral  occurs  very  widely  spread.  It  is  found 
in  veins  associated  with  quartz  (SiOs),  calcite  (CaCO-j),  barite  (BaSOi) 
or  fluorite  (CaF2)  and  various  sulphides,  especially  the  zinc  sulphide, 
sphalerite;    in  irregular  masses  filling  clefts  and  cavities  in  limestone; 


SULPHIDES,  TELLURIDES,   ETC.  83 

in  beds,  and  in  stalactites  and  other  forms  characteristic  of  water 
deposits. 

It  occurs  also  as  pseudomorphs  after  pyromorphite — the  lead  phos- 
phate. The  form  that  occurs  in  veins  is  often  silver  bearing,  while  that 
in  limestone  is  usually  free  from  silver. 

Localities. — Galena  is  mined  in  Cornwall  and  in  Derbyshire,  Eng- 
land; in  the  Moresnet  district,  Belgium;  at  various  places  in  Silesia, 
Bohemia,  Spain  and  Australia.  In  the  United  States  it  occurs  in  veins  at 
Lubec,  Me.,  at  Rossie,  St.  Lawrence  Co.,  N.  Y.,  at  Phoenixville,  Penn.,  at 
Austin's  Mines  in  Wythe  Co.,  Va.,  and  at  many  other  places.  It  is 
mined  for  silver  in  Mexico;  at  Leadville,  Colo.;  at  various  points  in 
Montana;  in  the  Cceur  d'Alene  region  in  Idaho  and  at  many  other  places 
in  the  Rocky  Mountain  region. 

The  most  extensive  galena  deposits  in  this  country  are  in  Missouri; 
in  the  corner  made  by  the  states  of  Wisconsin,  Illinois  and  Iowa;  and 
in  Cherokee  Co.,  Kansas.  In  these  districts  the  galena,  associated 
with  sphalerite  (ZnS),  pyrite  (FeS2),  smithsonite  (ZnCOa),  calamine 
((ZnOH)2Si03),  cerussite  (PbCOa),  calcite  (CaCCte)  and  other  minerals, 
fills  cavities  in  limestone. 

Extraction  of  Lead  and  Silver  from  Galena. — The  ore  is  first  crushed 
and  concentrated  by  mechanical  or  electrostatic  methods,  and  the 
concentrates  are  roasted  to  convert  them  into  oxides  and  sulphates. 
The  mass  is  then  heated  without  access  of  air,  sulphur  dioxide  being 
driven  off,  leaving  metallic  lead  carrying  impurities,  cr  a  mixture  of 
lead  and  silver. 

The  processes  employed  in  refining  the  impure  lead  vary  with  the 
nature  of  the  impurities. 

Uses. — Galena  is  employed  to  some  extent  in  glazing  common 
stoneware.  It  is  also  used  in  the  preparation  of  white  lead  and  other 
pigments.  As  has  alrcr.c!y  been  stated,  it  is  the  most  important  ore  of 
lead  and  a  very  important  ore  of  silver. 

The  metal  lead  finds  many  uses  in  the  arts.  Its  most  common 
use  is  for  piping.  Its  alloys,  type  metal,  pewter  and  babbitt  metal 
have  already  been  referred  to  (p.  74).  Solder  is  an  alloy  of  tin  and  lead; 
Wood's  metal  a  mixture  of  lead,  bismuth,  tin  and  cadmium.  The  spe- 
cial characteristic  of  Wood's  alloy  is  its  low  fusion  point  (700). 

Production. — The  total  production  of  galena  by  the  different  coun- 
tries of  the  world  cannot  be  given,  but  the  world's  production  of  lead 
in  191 2  was  1,277,002  short  tons.  The  total  quantity  of  lead  pro- 
duced by  the  United  States  from  domestic  ores  in  the  same  year  was 
about  4i5>395  tens,  valued  zt  $37,385,550.     Most  of  this  was  obtained 


g4  DESCRIPTIVE  MINERALOGY 

from  galena.  About  171,037  tons  were  soft  lead,  smelted  from  ores 
mined  mainly  for  their  lead  and  zinc  contents,  and  the  balance  from 
ores  mined  partly  for  their  silver.  The  importance  of  galena  as  an  ore 
of  silver  may  be  appreciated  from  the  fact  that  of  the  $39,197,000 
worth  of  this  metal  produced  in  the  United  States  during  191 2,  silver 
to  the  value  of  about  $12,000,000  was  obtained  from  lead  ores  or  from 
mixtures  of  lead  and  zinc  ores. 

Altaite  (PbTe)  and  clausthalite  (PbSe)  both  resemble  galena  in 
appearance.  Both  occur  commonly  in  fine-grained  masses,  but  they 
are  also  found  in  cubic  crystals.  Altaite  is  tin-white,  tarnishing  to 
yellow  or  bronze,  and  clausthalite  is  lead-gijay.  Their  hardness  is  2.5-3 
and  specific  gravity  about  8.1.  They  are  associated  with  silver  and  lead 
compounds  principally  in  the  silver  mines  of  Europe  and  South  America. 
Altaite  is  known  also  from  several  mines  in  California,  Colorado  and 
North  Carolina.  They  are  distinguished  from  one  another  and  from 
galena  by  the  tests  for  Te  and  Se. 

CHALCOCITE  GROUP 

The  chalcocite  group  includes  four  or  five  cuprous  and  argentous 
sulphides,  selenides  and  tellurides.  They  all  crystallize  in  the  ortho- 
rhombic  system  (rhombic  bipyramidal  class)  often  with  an  hexagonal 
habit,  and  are  isomorphous.  The  best  known  members  of  the  group 
are  chalcocite  (CU2S)  and  stromeyerite  (Cu-Ag)2S,  but  only  the  first- 
named  is  common.  Although  these  minerals  are  orthorhombic,  never- 
theless CU2S  is  known  to  exist  also  in  isometric  crystals,  in  which  form 
it  is  isomorphous  with  argentite.  Moreover,  stromeyerite  is  an  iso- 
morphous mixture  of  Ag2S  and  CU2S.  Therefore,  it  is  inferred  that 
CU2S  and  Ag2S  are  isomorphous  dimorphs. 

Chalcocite  (Cu2S) 

Chalcocite  (CU2S),  the  cuprous  sulphide,  is  an  important  ore  of 
copper  though  by  no  means  as  widely  spread  as  the  iron-copper  sul- 
phide, chalcopyrite.  It  is  usually  found  in  black  masses  with  a  dull 
metallic  luster  and  as  a  black  powder,  though  frequently  also  in  crys- 
tals. It  is  a  common  constituent  of  the  enrichment  zone  of  many  veins 
of  copper  ores. 

The  best  analyses  of  chalcocite  agree  closely  with  the  formula 
given  above,  requiring  the  presence  of  20.2  per  cent  of  sulphur  and 
79.8  per  cent  of  copper.  Iron  and  silver  are  often  present  in  the  mineral 
in  small  quantity. 


SULPHIDES,   TELLURIDES,   ETC.  85 

In  crystallization  chalcocite  is  orthorhombic  (rhombic  bipyramidal 
class)  with  the  axial  ratio  .5822  :  1  :  .9701.  Its  crystals  contain  as 
their   predominant    forms   oP(ooi),   ooP(no),    «Po&(oio),    P(ni), 

a  series  of  prisms  cf  the  general  symbol  — P(nA),   and  several  bra- 

chydomes.  Many  cf  the  crystals  are  elongated  parallel  to  H,  and 
others  are  so  developed  as  to  possess  an  hexagonal  habit  (Fig.  31). 
Twins  are  common  according  to  several  laws.  When  the  twinning  plane  is 
JP  (n a)  the  twins  are  usually  cruciform  (Fig.  31).  The  zone  001—  010 
is  often  striated  through  oscillatory  combinations.  iioaiio=6o°  25'. 
The  cleavage  of  chalcocite  is  indistinct,  its  fracture  is  conchoidal. 
Its  hardness  is  2.5-3  an^  density  about  5.7.     Its  streak,  like  its  color, 


Fie.  31.  Fig.  3a. 

Fie.  31.— Chalcocite  Crystal.    oP,  001  (c);    °oP«,  010(b);    »P,  no  (m);   iPw  , 

021  id);    JP«,023  («);   P.  i"  (?)andiP,  113  «■ 

Fio.  31.— Complex  Chalcocite  Twin,  with  »P,  no  (m)  and  )P,  112  {»)  the  Twinning 

Planes. 

is  nearly  black,  but  exposed  surfaces  are  often  tarnished  blue  or  green, 
probably  through  the  production  of  thin  films  of  other  sulphides  like 
covelUte  (CuS),  chalcopyrite  (FeCuSa),etc.  The  mineral  is  an  excel- 
lent conductor  of  electricity. 

In  the  open  tube  or  on  charcoal  chalcocite  melts  and  yields  sul- 
phurous fumes. 

When  mixed  with  NazCOs  and  heated  a  copper  globule  is  produced. 
The  mineral  dissolves  in  nitric  acid  with  the  production  of  a  solution 
that  yields  the  test  for  copper. 

Upon  exposure  to  the  air  chalcocite  changes  readily  to  the  oxide, 
cuprite  (CU2O),  and  the  carbonates,  malachite  and  azurite.  In  the 
presence  of  silicious  solutions  it  may  give  rise  to  the  silicate,  chrysocolla 
(p.  441). 

A  pseudomorph  of  chalcocite  after  galena  is  known  as  harrisiU. 


86  DESCRIPTIVE  MINERALOGY 

It  occurs  at  the  Canton  Mine  in  Georgia  and  in  the  Polk  Co.  copper 
mines  in  Tennessee.  Pseudomorphs  after  many  other  copper  min- 
erals are  common. 

Chalcocite  is  recognized  by  its  color  and  crystallization.  Massive 
varieties  are  distinguished  from  or  gentile  by  greater  brittleness  and  the 
reaction  for  copper;  from  bornite  (CuaFeSa)  by  the  fact  that  it  is  not 
magnetic  after  roasting. 

Syntheses. — Crystals  of  chalcocite  have  been  made  in  many  ways, 
more  particularly  by  heating  the  vapors  of  CuCk  and  H2S,  and  by 
gently  warming  CU2O  in  H2S.  Measurable  crystals  have  been  observed 
on  old  bronze  that  has  been  immersed  in  the  waters  of  hot  springs  for 
a  long  time. 

Occurrence,  Localities  and  Origin. — The  mineral  is  a  common  prod- 
uct of  the  alteration  of  other  copper  compounds  in  the  zone  of  secondary 
enrichment  of  sulphide  veins.  It  is  therefore  present  at  most  localities 
of  copper  minerals.  One  of  the  best  known  occurrences  is  Butte, 
Mont. 

Fine  crystals  of  chalcocite  occur  in  veins  and  beds  at  Redruth  and 
at  other  places  in  Cornwall,  England;  at  Bristol  in  Connecticut,  and 
at  Joachimthal  in  Bohemia.  The  massive  variety  is  known  at  many 
places.  In  the  United  States  it  occurs  in  red  sandstone  at  Cheshire 
in  Connecticut.  It  is  found  also  in  large  quantities  near  Butte  City  in 
Montana,  and  in  Washoe  and  other  counties  in  Nevada,  and  indeed 
in  the  veins  of  most  copper  producing  mines.  In  Canada  it  is  present 
with  chalcopyrite  and  bornite  at  Acton,  Quebec,  and  at  several  places 
in  Ontario  north  of  Lake  Superior. 

Extraction  of  Copper. — Chalcocite  rarely  occurs  alone  in  large 
quantity.  In  ores  it  is  usually  mixed  with  other  compounds  of  copper, 
and  is  treated  with  them  in  extracting  the  metal  (see  p.  133). 

Stromeyerite  ((Ag-Cu)2S)  is  usually  massive,  but  it  occurs  also  in 
simple  and  twinned  crystals  similar  to  those  of  chalcocite.  Their  axial 
ratio  is  .5822  :  1  :  .9668,  almost  identical  with  that  of  chalcocite.  The 
mineral  is  opaque  and  metallic.  Its  color  and  streak  are  dark  steel- 
gray.  Its  hardness  is  2.5-3  and  density  about  6.2.  It  is  soluble  in 
nitric  acid.  It  occurs  associated  with  other  sulphides  in  the  ores  of 
silver  and  copper  mines  at  Schlangenberg,  Altai;  Kupferberg,  Silesia; 
Coquimbo,  Copiapo,  and  other  places  in  Chile,  and  in  a  few  mines  in 
California,  Arizona,  and  Colorado. 


SULPHIDES,  TELLURIDES,  ETC.  87 


BLENDE  GROUP 

The  blende  group  of  minerals  comprises  a  series  of  compounds  whose 
general  formula  like  that  of  the  galena  group  is  RQ.  In  the  blendes  R 
stands  for  Zn,  Cd,  Mn,  Ni  and  Fe  and  Q  for  S,  Se  and  Te. 

The  blendes  are  all  transparent  or  translucent  minerals  of  a  lighter 
color  than  galena.  They  constitute  an  isodimorphous  group  of  a  dozen 
or  more  members  crystallizing  in  the  tetrahedral  division  of  the  regular 
system  (hextetrahedral  class),  and  in  hemimorphic  holohedral  forms  of 
the  hexagonal  system  (dihexagonal-pyramidal  class).  The  group  may 
be  divided  into  two  subgroups  known  respectively  as  the  sphalerite 
and  the  wurtzite  groups. 

SPHALERITE   DIVISION 

The  most  important  member  of  this  division  of  the  blende  group  is 
the  mineral  sphalerite.  This,  like  the  other  less  well  known  members, 
crystallizes  in  the  hemihedral  division  of  the  regular  system  with  various 
tetrahedrons  as  prominent  forms.  The  other  members  of  the  group 
are  alabandite  (MnS),  and  an  isomorphous  mixture  of  FeS  and  NiS, 
pentlandite. 

Sphalerite  (ZnS) 

Sphalerite,  one  of  the  very  important  zinc  ores  and  one  of  the  most 
interesting  minerals  from  a  crystallographic  standpoint,  occurs  in  amor- 
phous and  crystalline  masses  and  in  handsome  crystals  and  crystal  groups. 
Botryoidal  and  other  imitative  masses  are  common. 

Pure  white  sphalerite  consists  of  67  per  cent  of  Zn  and  23  per  cent  of 
sulphur.  The  colored  varieties  usually  contain  traces  of  silver,  iron, 
cadmium,  manganese  and  other  metals.  Sometimes  the  proportion  of 
the  impurities  is  so  large  that  the  mineral  containing  them  is  regarded  as 
a  distinct  variety.    Two  analyses  of  American  sphalerites  are  as  follows: 

S  Zn       Cd      Fe  Total 

Franklin  Furnace,  N.  J 32 . 22        67 .46      tr      ...  99.68 

Joplin,  Mo 32-93        66.69     •••      -42  100.04 

The  hemihedral  condition  of  sphalerite  is  shown  in  the  predominance 
of  tetrahedrons  among  its  crystal  forms  and  by  the  symmetry   of    its 

etched  figures  (Fig.  33).    Its  most  common  forms  are  —  - — (321)  and 

2 

2O  lO 

other  hextetrahedrons,   d= — (221),  —  (331)  and  other  deltoid-dodeca- 

2  2 


88  DESCRIPTIVE  MINERALOGY 

hedrons  and  ±303(311)  and  other  tris tetrahedrons.  In  addition, 
00  O  00  (100)  and  00  0(i  10)  are  quite  common  (Fig.  34).  Twins  are 
abundant.  Their  twinning  plane  is  O  and  their  composition  face  either 
0  (Fig.  35),  or  a  plane  perpendicular  to  this.  Through  twinning,  the 
crystals  often  assume  a  rhombohedral  habit. 

The  cleavage  of  sphalerite  is  perfect  parallel  to  00  O(no).  From  a 
compact  mass  of  the  mineral  a  fairly  good  dodecahedron  may  some- 
times be  split.  Its  fracture  is  conchoidal.  When  pure  the  mineral  is 
transparent  and  colorless.  As  usually  found,  however,  it  is  yellow, 
translucent  and  black,  brown,  or  some  shade  of  red.  Its  streak  is 
brownish,  yellow  or  white.    The  yellow  masses  look  very  much  like 


FlO.  33.— Telrahedral  Crystal  of  Sphalerite  Bounded  by  »0»(ioi)  and  ±0  (11 1 
and  ill),  Illustrating  the  Fact  that  Its  Octahedral  Faces  Fall  into  Two  Groups. 

Fie.  34.— Sphalerite  Crystal:    «0,  no  (rf),  and+— ^,  311  (m). 

Fig.  35.— Sphalerite  Octahedron  Twinned  about  0(m). 

lumps  of  rosin.  The  hardness  of  sphalerite  is  between  3.5  and  4,  and  its 
density  about  4.  Its  luster  is  resinous.  The  mineral  is  difficultly  fusible, 
and  is  a  nonconductor  of  electricity.  Its  index  of  refraction  («)  for 
yellow  light  is  2.369. 

Sphalerite  when  powdered  always  yields  tests  for  sulphur  under 
proper  treatment.  On  charcoal  it  volatilizes  slowly,  coating  the  coal 
with  a  yellow  sublimate  when  hot,  turning  white  on  cooling.  When 
moistened  with  a  dilute  solution  of  cobalt  nitrate  and  heated  in  the 
reducing  flame,  the  white  coating  of  ZnO  turns  green.  The  mineral  dis- 
solves in  hydrochloric  acid,  yielding  sulphuretted  hydrogen. 

By  oxidation  sphalerite  changes  into  the  sulphate  of  zinc,  and  by 
other  processes  into  the  silicate  of  zinc,  calamine,  or  the  carbonates, 
smithsonite  and  hydrozincite. 


SULPHIDES,  TELLURIDES,  ETC.  89 

Syntheses. — Sphalerite  crystals  have  been  made  by  the  action  of  H2S 
upon  zinc  chloride  vapor  at  a  high  temperature.  They  are  also  often 
produced  in  the  flues  of  furnaces  in  which  ores  containing  zinc  and  sul- 
phur are  roasted. 

Occurrence  and  Origin. — Sphalerite  occurs  disseminated  through  lime- 
stone, in  streaks  and  irregular  masses  in  the  same  rock,  and  in  veins  cut- 
ting crystalline  and  sedimentary  rocks.  It  is  often  associated  with 
galena.  The  material  in  the  veins  is  often  crystallized.  Here  it  is  asso- 
ciated with  chalcopyrite  (CuFeS2),  fluorite  (CaF2),  barite  (BaSCU), 
siderite  (FeCOa),  and  silver  ores.  When  in  veins  it  is  in  some  cases  the 
result  of  ascending  hot  waters  and  in  other  cases  the  product  of  down- 
ward percolating  meteoric  water.  Much  of  the  disseminated  ore  is  a 
metamorphic  contact  deposit. 

Localities. — Crystallized  sphalerite  is  found  abundantly  at  Alston 
Moor,  Cumberland,  England;  at  various  places  in  Saxony;  in  the  Bin- 
nenthal,  Switzerland;  at  Broken  Hill,  N.  S.  Wales,  and  in  nearly  all 
localities  for  galena.  Handsome,  transparent,  cleavable  masses  are 
brought  from  Pilos  de  Europa,  Santander,  Spain.  Stalactites  are 
abundant  near  Galena,  111. 

The  principal  deposits  of  economic  importance  in  America  are  those 
in  Iowa,  Wisconsin,  Missouri  and  Kansas,  where  the  sphalerite  is  asso- 
ciated with  other  zinc  compounds  and  with  galena  forming  lodes  in 
limestone,  and  at  the  silver  and  gold  mines  of  Colorado,  Idaho  and  Mon- 
tana, 

Extraction  of  the  Metal. — In  order  to  obtain  the  metal  from  sphalerite, 
the  ore  is  usually  first  concentrated  by  flotation  or  other  mechanical 
processes.  The  concentrates  are  then  converted  into  the  oxide  by  roast- 
ing and  the  impure  oxide  is  mixed  with  fine  coal  and  placed  in  clay  retorts 
opening  into  a  condenser.  These  are  gradually  heated.  The  oxide  is 
reduced  to  the  metal,  which  being  volatile  distils  over  into  the  con- 
denser, where  it  is  safely  caught.  Other  processes  are  based  on  wet 
chemical  methods. 

Uses  of  Zinc. — Zinc  is  used  extensively  in  galvanizing  iron  wire  and 
sheets.  It  is  also  employed  in  the  manufacture  of  important  alloys 
such  as  brass,  and  in  the  manufacture  of  zinc  white,  which  is  the  oxide 
(ZnO),  and  other  pigments.  A  solution  of  the  chloride  is  used  for  pre- 
serving timber.  Argentiferous  zinc  is  the  source  of  a  considerable  quan- 
tity of  silver. 

Production. — The  figures  showing  the  quantity  of  sphalerite  pro- 
duced in  the  zinc-producing  countries  are  not  available.  The  total 
amount  of  metallic  zinc  produced  in  the  year  191 2  was  1,070,045  tons, 


90  DESCRIPTIVE  MINERALOGY 

valued  at  $44,699,166,  of  which  the  United  States  produced  from  domestic 
ores  323,907  tons,  and  in  addition  used,  in  the  making  of  zinc  compounds, 
about  55,000  tons.  Of  this  aggregate,  Missouri  produced  about  149,560 
tons.  Most  of  the  metal  was  obtained  from  sphalerite,  but  a  large 
part  came  from  other  ores.  The  quantity  of  silver  produced  in  refining 
zinc  ores  was  664,421  oz.,  valued  at  $408,619. 

Alabandite  (MnS)  is  isomorphous  with  sphalerite.  It  usually 
occurs,  however,  in  dense  granular  aggregates  of  an  iron-gray  color. 
Its  streak  is  dark  green.  It  is  opaque  and  brittle.  Its  hardness  is  3-4 
and  density  3.9.  It  is  not  an  electrical  conductor.  When  heated  on 
charcoal  in  the  reducing  flame  it  changes  to  the  brown  oxide  of  man- 
ganese and  finally  melts  to  a  brown  slag.  It  is  soluble  in  dilute  HC1 
with  the  evolution  of  H2S.  Alabandite  occurs  with  other  sulphides  at 
Kapnik,  Hungary;  at  Tarma,  Peru;  at  Puebla,  Mexico,  and  in  the 
United  States  at  Tombstone,  Arizona,  and  on  Snake  River,  Summit  Co., 
Colorado. 

Pentlandite  ((Fe  •  Ni)S)  may  belong  to  this  group.  Iron  is  frequently 
found  in  crystallized  sphalerite.  Its  sulphide,  therefore,  may  be  isomor- 
phous with  sphalerite,  in  which  case  pentlandite,  which  is  probably  an 
isomorphous  mixture  of  NiS  and  FeS,  would  also  belong  in  the  sphal- 
erite group.  The  mineral  occurs  in  light  bronzy  yellow,  granular  masses 
with  a  distinct  octahedral  cleavage,  a  hardness  of  3.5-5  and  a  density  of 
4.6.  It  is  a  nonconductor  of  electricity.  Pentlandite  occurs  with 
chalcopyrite  (CuFeS2)  and  pyrrhotite  (FeySg),  at  Sudbury,  Ontario, 
where  it  is  probably  the  constituent  that  furnishes  most  of  the  nickel 
(see  p.  92). 

It  is  distinguished  from  pyrrhotite,  which  it  resembles  in  appearance, 
by  its  cleavage  and  the  fact  that  it  is  not  magnetic.  Moreover,  it 
weathers  to  a  brassy  yellow  color,  while  pyrrhotite  weathers  bronze. 

WURTZITE   DIVISION 

The  wurtzite  group  comprises  only  two  or  three  members,  wurtziie 
(ZnS),  greenockite  (CdS),  and  possibly  pyrrhotite  (FenSn+1).  All  crys- 
tallize in  the  holohedral  division  of  the  hexagonal  system  and  the  first 
two  are  unquestionably  hemimorphic  (dihexagonal  pyramidal  class). 
Pyrrhotite  is  the  most  common. 

Wurtzite  (ZnS)  is  one  of  the  dimorphs  of  ZnS,  sphalerite  being  the 
other.    It  occurs  in  brownish  black  crystals,  in  masses  and  in  fibers. 


SULPHIDES,  TELLURIDES,   ETC.  91 

Its  crystals  are  combinations  of  ooP(ioTo)  with  2P(202i)  and 
oP(oooi)  at  one  end,  and  a  series  of  steeper  pyramids  at  the 
other.     Their  axial  ratio  is  i  :  .8175.    The  angle  10T1 A 01^1  =  40°  9'; 

2P(022l)  A2P(022l)  =  52°  27'. 

The  mineral  is  brownish  black  to  brownish  yellow  and  its  streak 
is  brown.  Its  hardness  is  between  3  and  4  and  its  sp.  gr.  is  about  4. 
It  conducts  electricity  very  poorly.  In  chemical  and  physical  prop- 
erties it  resembles  sphalerite.  Its  crystals  have  been  produced  by 
fusing  a  mixture  of  ZnS04,  fluorite  and  barium  sulphide.  They  are 
frequently  observed  as  furnace  products. 

Wurtzite  occurs  as  crystals  at  the  original  Butte  Mine,  Butte, 
Montana,  and  in  a  mine  near  Benzberg,  Rhenish  Prussia,  at  both 
places  associated  with  sphalerite.  They  also  occur  with  silver  ores  near 
Oruro  and  Chocaya,  Bolivia,  and  near  Quispisiza,  Peru. 

Greenockite. — Greenockite  (CdS)   is  completely  isomorphous  with 
wurtzite.     Its  crystals  have    an   axial   ratio 
1  :  .8109.      In    general    habit  they  are   like 
those  of  wurtzite  but  they  contain  many  more 
planes  (Fig.   36).     The  angle  io7iaoi^i  = 
390  58'.    Crystals  are  rare  and  small.    The 
mineral  usually  occurs  as  a  coating  on  other 
minerals,  especially  sphalerite.     Its  color  is 
honey  to  orange-yellow,  its    streak  orange-    Fig.  36.— Greenockite  Crys- 
yellow,  and  its  luster  glassy  or  resinous.     It       taL    00  p,  ioTo^(m);   2P, 
is  transparent  or    translucent  and  is  brittle.       2021  («);  P,  ion  (*),  and 

Its  hardness  is  3-3.5  and  density  about  4.0.       ?  '    -/f .    Y.  e  orm 

0  °  J  .  J  "*  *         JP,  1012  (i)  is  often  pres- 

Its    index    of    refraction    0  =  2.688.      When       Cnt  at  the  upper  end  of 

heated  in  the  closed  tube  it  becomes  carmine,       the  crystals.) 

but  it  changes  to  its  original  color  on  cooling. 

It  yields  the  usual  reactions  for  sulphur  and  cadmium,  and  dissolves 

in  HC1,  yielding  H2S. 

Crystals  have  been  obtained  by  melting  a  mixture  of  CdO,  BaS, 
and  CaF2,  and  by  heating  cadmium  in  an  atmosphere  of  H2S  to  near 
fusing  point.  The  mineral  is  a  common  furnace  product.  Greenockite 
crystals  occur  with  prehnite  at  Bishoptown,  Scotland,  and  as  coatings 
on  sphalerite  in  the  zinc  regions  of  Missouri  and  Arkansas,  and  at 
Friedensville,  Pennsylvania. 


92  DESCRIPTIVE  MINERALOGY 


Pyrrhotite  (FenSn+i) 

Pyrrhotite,  or  magnetic  pyrite,  occupies  the  anomalous  position 
of  being  one  of  the  most  important  ores  of  nickel,  whereas  it  is  essen- 
tially a  sulphide  of  iron.  The  name  is  really  applied  to  a  series  of 
compounds  whose  composition  ranges  between  FesSe  and  FeieSiy. 
The  crystallized  material  is  in  some  cases  FerSg,  and  in  others,  FenSi2. 
It  is  probably  a  solid  solution  of  FeS2  or  S  in  the  sulphide  of  iron  (FeS). 
As  usually  found,  pyrrhotite  is  in  bronze-gray  granular  masses,  that 
tarnish  rapidly  to  bronze  on  exposure  to  the  air.  Good  crystals  of 
the  mineral  are  rare. 

Analyses  of  pyrrhotite  vary  widely.  The  percentages  of  Fe  and  S 
corresponding  to  FeySg  are  Fe,  60.4;  S,  39.6,  and  those  corresponding 
to  FenSi2  are  Fe,  61.6;  S,  38.4.  Much  of  the  mineral  contains  in  addi- 
tion to  the  iron  and  sulphur  sufficient  nickel  to  render  it  an  ore  of  this 
metal,  but  it  is  probable  that  the  nickel  is  present  in  pentlandite  (see 
p.  90)  or  some  other  nickel  compound  embedded  in  the  pyrrhotite. 

Analyses  of  pyrrhotite  from  various  localities  are: 

S  Fe  Co  Ni  Total 

Schneeberg,  Saxony 39 .  10  61.77  tr            IO° •  87 

Brewster,  N.  Y 37 . 98  61 . 84  ...  .25  100.07 

Sudbury,  Ontario 38. 91  56.39  ....  4.66  99 .96 

Gap  Mine,  Penn 38. 59  55 .82  5.59  100.00 

The  few  crystals  of  pyrrhotite  known  are  distinctly  hexagonal  in 

habit  with  a  :  c=i  :  1.7402.    They  are  com- 
monly tabular  or  acutely  pyramidal,  but  it 
has  not  been  established  that  they  are  hemi- 
m—*     morphic,  although  the  almost  universal  pres- 
ence of  FeS   in   crystals   of  wurtzite  would 
Fig.  37.-Pyrrhotite  Crystal.   ^^^  that  the  two  substances  are  isomor- 
oP,  0001  (c);  P,  1011  (5);      ,  „,,      .    ,     ,  .    .  ,         , 

P    4041   (u)    and   00  p    Ph°us'    The  tabular  crystals  possess  a  broad 

10T0  (m).       '  basal  plane,  which  surmounts  hexagonal  prisms 

ooP(ioTo)  and  ooP2(ii2o),  and  a  series  of 
pyramids,  of  which  2P(202i),  iP(ioi2),  P(ioTi)  and  P2(ii22)  are  the 
most  frequent.     (Fig.  37.)      The  angle  10T1  AoiTi  =  53°  n'. 

The  cleavage  of  pyrrhotite  is  not  always  equally  distinct.  When 
marked  it  is  parallel  to  00  P2 (11 20).  There  is  also  often  a  parting 
parallel  to  the  base.  Its  fracture  is  uneven.  The  mineral  is  brittle. 
It  is  opaque,  and  has  a  metallic  luster.    Its  color  varies  between  bronze- 


0 


SULPHIDES,  TELLURIDES,  ETC.  93 

yellow  and  copper-red,  and  its  streak  is  grayish  black.  Its  hardness  is 
a  little  less  than  4  and  its  density  about  4.5.  All  specimens  are  magnetic 
but  the  magnetism  varies  greatly  in  intensity,  being  at  a  maximum  in 
the  direction  of  the  vertical  axis.  The  mineral  is  a  good  conductor  of 
electricity. 

Pyrrhotite  gives  the  usual  reactions  for  iron  and  sulphur,  and  some- 
times, in  addition,  the  reactions  for  cobalt  and  nickel.  It  is  decom- 
posed by  hydrochloric  acid  with  the  evolution  of  H2S,  which  may 
easily  be  detected  by  its  odor. 

From  the  many  sulphides  more  or  less  closely  resembling  pyrrhotite 
in  appearance,  this  mineral  may  easily  be  distinguished  by  its  color 
and  density  and  by  its  magnetism. 

Syntheses. — Crystals  may  be  obtained  by  heating  iron  wire  or 
Fea04,  or  dry  FeCb  to  redness  in  an  atmosphere  of  dry  H2S  and  by 
heating  Fe  in  a  closed  tube  with  a  solution  cf  f  eCfe  saturated  with 
H2S. 

Occurrence,  Localities  and  Origin. — Pyrrhotite  occurs  completely 
filling  vein  fissures,  and  also  as  crystals  embedded  in  other  minerals 
constituting  veins.  It  occurs  also  as  impregnations  in  various  rocks 
and  as  a  segregation  in  the  coarse-grained  basic  rock  known  as  norite, 
where  it  is  believed  to  have  separated  from  the  magma  producing  the 
rock.  It  may  also  in  some  cases  be  a  product  of  metamorphism  on  the 
borders  of  igneous  intrusions. 

It  is  found  at  Andreasberg,  Harz;  Bodenmais,  Bavaria;  Minas 
Geraes,  Brazil;  various  points  in  Norway  and  Sweden,  and  on  the 
lavas  of  Vesuvius.  In  North  America  crystals  occur  at  Standish,  Maine; 
at  Trumbull,  Monroe  Co.,  N.  Y.;  and  at  Elizabethtown,  Ontario. 
The  mineral  has  been  mined  at  Ducktown,  Tenn.;  at  Ely,  Vermont, 
and  at  Gap  Mine,  Lancaster  Co.,  Penn. 

Its  mines  at  present,  however,  are  at  Sudbury,  in  Ontario,  where  the 
mineral  is  associated  with  magnetite,  chalcopyrite  and  pentlandite 
((Fe-Ni)S)  on  the  lower  border  of  a  great  mass  of  igneous  rock  (norite). 
Besides  these  there  are  present  also  embedded  in  the  pyrrhotite 
small  quantities  of  other  minerals,  so  that  the  ore  as  mined  is  very 
complex. 

Pyrrhotite  is  sometimes  found  altered  to  pyrite,  to  limonite  and  to 
siderite  (FeCOa). 

Extraction. — Pyrrhotite  is  crushed  and  roasted  to  drive  off  the 
greater  portion  of  the  sulphur.  It  is  then  placed  in  a  furnace  and 
smelted  with  coke  and  quartz.  The  nickel,  copper  and  some  of  the 
iron,  together  with  some  of  the  fused  sulphides,  collect  as  a  matte  in  the 


94  DESCRIPTIVE  MINERALOGY 

bottom  of  the  furnace  from  which  it  is  withdrawn  from  time  to  time. 
The  matte  is  riext  roasted  to  transform  the  iron  it  contains  into  oxides 
and  the  remaining  nickel  and  copper  are  separated  by  patented  or  secret 
methods. 

Uses. — The  mineral  is  sometimes  worked  for  the  sulphur  it  con- 
tains. Its  principal  use,  however,  is  as  a  source  for  nickel,  nearly  all  of 
this  metal  used  in  America  coming  from  the  nickeliferous  variety  found 
at  Sudbury,  Ontario. 

The  metal  nickel  has  come  into  extensive  use  in  the  past  few  years 
in  connection  with  the  manufacture  of  armor  plate  for  warships.  The 
addition  of  a  few  per  cent  of  nickel  to  steel  hardens  it  and  increases 
its  strength  and  elasticity. 

Nickel  is  also  extensively  used  in  nickel-plating  and  in  the  manufac- 
ture of  alloys.  German  silver  is  an  alloy  of  nickel,  copper  and  zinc.  The 
nickel  currency  of  the  United  States  contains  about  25  per  cent  Ni  and 
75  per  cent  Cu.  Monel  metal  is  a  silver- white  alloy  containing  about 
75  per  cent  Ni,  1  per  cent  Fe  and  29  per  cent  Cu.  It  is  stronger  than 
ordinary  steel,  takes  a  brilliant  finish  and  is  impervious  to  acids.  It  is 
made  directly  at  Sudbury,  Ont.,  by  smelting. 

Production. — The  production  of  pyrrhotite  and  chalcopyrite  (CuFeS) 
at  the  Sudbury  mines  in  191 2  amounted  to  737,584  short  tons.  The 
value  of  the  matte  produced  was  $6,303,102,  and  the  value  of  nickel  con- 
tained in  it  was  about  $16,000,000.  About  half  of  the  nickel  was  used 
in  America;  the  remainder,  amounting  to  $8,515,000,  was  exported,  after 
being  refined  in  the  United  States.  Formerly  the  United  States  pro- 
duced a  considerable  quantity  of  nickel  from  domestic  ores,  most  of 
it  from  pyrrhotite,  but  the  mines  have  been  closed  down  within  the  past 
few  years.  It  is,  however,  produced  as  a  by-product  in  the  refining 
of  copper  ores  to  the  amount  of  about  325  tons  annually,  This  is  worth 
about  $260,000  (see  also  p.  400). 

MILLERITE  GROUP 

This  group  comprises  sulphides,  arsenides  and  antimonides  of  nickel. 
It  includes  the  minerals  mUlerite  (NiS) 9niccolite  (NiAs),  arite  (Ni(Sb-  As)) 
breithauptite  (NiSb)  and  a  few  others.  Of  these  only  milled te  and  nic- 
colite  are  at  all  common.  The  minerals  all  crystallize  in  the  hexagonal 
system,  possibly  in  the  rhombohedral  division  (ditrigonal  scalenohedral 
class).  Well  defined  crystals  are,  however,  rare  and  often  capillary  so 
that  their  symmetry  has  not  been  determined  with  certainty. 


SULPHIDES,  TELLURIDES,  ETC.  95 


Millerite  (NiS) 

Millerite  is  easily  recognized  by  its  brass-yellow  color.  It  occurs 
most  frequently  in  slender  hair-like  needles,  often  aggregated  into  tufts 
or  radial  groups,  or,  woven  together  like  wads  of  hair,  forming  coatings 
on  other  minerals. 

Pure  millerite  contains  35.3  per  cent  sulphur  and  64.6  per  cent  nickel. 
It  frequently  contains  also  a  little  Co  and  Fe. 

Crystals  are  thin,  acicular  or  columnar  with  prismatic  and  rhom- 
bohedral  faces  predominating,  and  an  axial  ratio  of  1  :  .330,  or  of  1  :  .9886 
if  the  rhombohedron  311(0331)  is  taken  as  the  ground  form. 

The  mineral  is  elastic.  Its  hardness  is  3-3.5  and  density  about  5.5. 
It  is  opaque  and  brassy  yellow.  Its  streak  is  greenish  black.  It  is  an 
excellent  conductor  of  electricity. 

The  mineral  yields  sulphurous  fumes  in  the  open  tube.  After  roast- 
ing it  gives,  with  borax  and  microcosmic  salt,  a  violet  bead  when  heated 
in  the  oxidizing  flame  of  the  blowpipe.  On  charcoal  with  Na2C03  it 
yields  a  magnetic  globule. 

Synthesis. — Bunches  of  yellow  acicular  crystals  of  NiS  have  been 
formed  by  treatment  of  a  solution  of  NiSO*  with  H2S,  under  pressure. 

Localities. — Millerite  occurs  as  long  acicular  crystals  in  cavities  in 
other  minerals  at  Joachim  thai,  in  Bohemia,  and  at  many  places  in 
Saxony.  In  the  United  States  it  forms  radiating  groups  in  cavities  in 
hematite  (Fe20s)  at  Antwerp,  N.  Y.  At  the  Gap  Mine,  Lancaster  Co., 
Penn.,  it  forms  coatings  on  other  minerals  and  at  St.  Louis,  Mo.  and 
at  Milwaukee,  Wis.,  it  occurs  in  delicate  tangled  tufts  in  geodes  in  lime- 
stone.  Nowhere  does  it  occur  in  sufficient  quantity  to  constitute  an  ore* 

Niccolite  (NiAs) 

Niccolite  usually  occurs  massive,  though  crystals  are  known.  It  is 
of  economic  importance  only  in  a  few  localities. 

Theoretically,  the  mineral  contains  56.10  per  cent  As  and  43.90  per 
cent  Ni,  but  as  usually  found  it  contains  also  Sb,  S,  Fe  and  often  small 
quantities  of  Co,  Cu,  Pb  and  Bi. 

Its  crystals,  which  are  rare,  are  hexagonal  and  hemimorphic  (prob- 
ably dihexagonal  pyramidal  class),  with  0:1=1:  .8194.  The  prism 
ooP(ioTo),  and  oP(oooi)  are  the  predominant  forms,  with  the 
pyramids  P(ioTi)    and  ^(5057)    less   well  developed.     The    angle 

io7i  aoi^i==4o°  i2'- 

The  mineral  is  pale  copper-red  and   opaque.     It  has  a  brownish 


fl»  DESCRIPTIVE  MINERALOGY 

black  streak.  Its  hardness  is  about  5  and  its  density  7.6.  The  surfaces 
of  nearly  all  specimens  are  tarnished  with  a  grayish  coating.  The  min- 
eral is  a  good  conductor  of  electricity. 

In  the  open  tube  niccolite  yields  arsenic  fumes  and  often  traces  of 
SO2.  On  charcoal  with  Na2COa  it  yields  a  metallic  globule  of  nickel. 
It  dissolves  in  HNO3  with  the  precipitation  of  AS2O3.  The  apple-green 
solution,  thus  produced,  becomes  sapphire-blue  on  addition  of  ammonia. 

Its  peculiar  light  pink  color  and  its  reactions  for  arsenic  and  nickel 
distinguish  niccolite  from  all  other  minerals,  except,  perhaps,  breit- 
hauptitCy  which,  however,  contains  antimony. 

Occurrence. — Niccolite  occurs  principally  in  veins  in  crystalline 
schists  and  in  metamorphosed  sedimentary  rocks,  associated  with  silver 
and  cobalt  sulphides  and  arsenides. 

Localities. — The  principal  locality  for  niccolite  in  North  America  is 
Cobalt,  Ontario,  where  it  is  found  with  native  silver  and  silver,  cobalt, 
and  other  nickel  compounds,  all  of  which  are  thought  to  have  been  de- 
posited by  hot  waters  emanating  from  a  mass  of  diabase.  In  Europe  it 
is  abundant  at  Joachimsthal  in  Bohemia,  and  at  a  number  of  other 
places  in  small  quantity. 

Although  rich  in  nickel,  the  mineral  is  not  used  as  an  ore  at  present, 
except  to  a  very  minor  extent,  most  of  the  nickel  of  commerce  being 
obtained  from  other  compounds  (see  p.  94). 

Breithauptite  (NiSb)  is  rare.  It  is  of  a  light  copper-red  color,  much 
brighter  than  that  of  niccolite,  and  its  streak  is  reddish  brown.  Its  hard- 
ness is  5.5  and  density  about  7.9.  Its  crystals  are  hexagonal  tables 
with  an  axial  ratio  1  :  1.294,  and  a  distinct  cleavage  parallel  to  oP(ooi). 
It  usually  occurs  in  dendritic  groups,  in  foliated  and  finely  granular 
aggregates  and  in  dense  masses.  It  is  a  frequent  furnace  product,  when 
ores  containing  Ni  and  Sb  are  smelted.  It  is  found  at  Andreasberg,  Harz ; 
at  Sarrabus,  in  Sardinia;  at  Cobalt,  Ont.,  and  at  a  few  other  places.  It 
is  distinguished  from  niccolite  by  its  deeper  color  and  its  content  of  Sb. 


Covellite  (CuS) 


Covellite,  or  indigo  copper,  is  the  cupric  sulphide,  chalcocite  being 
the  corresponding  cuprous  salt.  It  is  called  indigo  copper  because  of 
the  deep  blue  color  of  its  fresh  fracture.  It  is  often  mixed  with  other 
copper  compounds  from  which  it  has  been  derived  by  alteration.  It 
usually  occurs  massive,  but  crystals  are  known.  It  is  an  unimportant 
ore  of  copper. 


SULPHIDES,  TELLURIDES,  ETC.  97 

The  theoretical  composition  of  the  mineral  is  33.56  per  cent  S, 
66.44  Per  cent  Cu.  It  usually,  however,  contains  also  a  little  iron  and 
often  traces  of  lead  and  silver. 

Crystals  of.  covellite  are  not  common.  They  are  hexagonal  with 
a  :  c=  1  :  3.972  and  their  habit  is  usually  tabular.  The  forms  observed 
are  oP(oooi),  00  P(ioTo),  P(ioTi)  and  JP(iol4).    10T1  Aoi^i  =  77°  42'. 

The  mineral  has  one  perfect  cleavage  parallel  to  oP(oooi).  In 
thin  splinters  it  is  flexible.  Its  hardness  is  1.5-2  and  density  about 
4.6.  Its  color  is  dark  blue  and  its  streak  lead-gray  to  black.  It  is 
opaque,  with  a  luster  that  is  sometimes  nearly  metallic,  but  more 
frequently  dull.    It  is  a  good  electrical  conductor. 

The  blowpipe  reactions  of  covellite  are  like  those  of  chalcocite,  with 
these  exceptions:  Covellite  burns  with  a  blue  flame  when  heated  on 
charcoal,  and  yields  a  sublimate  of  sulphur  in  the  closed  tube. 

Covellite  is  distinguished  from  other  minerals  than  chalcocite  by 
its  reactions  for  Cu  and  S  and  the  absence  of  reactions  for  Fe.  It  is 
distinguished  from  chalcocite  by  its  color  and  density  and  by  the  fact 
that  it  ignites  on  charcoal. 

Syntheses. — The  treatment  of  green  copper  carbonate  with  water 
and  H2S  in  a  closed  tube  at  8o°-9o°  yields  small  grains  of  covellite. 
The  mineral  has  also  been  produced  by  the  action  of  H2S  upon  vapor 
of  Q1Q2,  and  by  treating  sphalerite  with  a  solution  of  copper  sulphate 
in  a  sealed  glass  tube  containing  CO2  at  a  temperature  of  i5o°-i6o° 
for  two  days. 

Localities  and  Origin. — The  mineral  is  comparatively  rare.  It  is 
abundant  in  Chile  and  Bolivia  and  at  Butte,  Mont.,  and  is  found  in 
crystals  on  the  lava  of  Vesuvius  and  elsewhere.  It  usually  occurs  as 
an  alteration  product  of  other  copper-sulphur  compounds,  especially  in 
the  zone  of  secondary  enrichment  of  copper  veins. 

Uses. — It  is  mined  with  other  compounds  and  used  as  a  source 
of  copper. 

CINNABAR  GROUP 

This  group  comprises  sulphides,  selenides  and  tellurides  of  mercury. 
The  group  is  dimorphous,  with  its  members  crystallizing  in  hemihedrons 
of  the  isometric  system  (hextetrahedrai  class)  and  in  tetartohedrons 
of  the  hexagonal  system  (trigonal  trapezohedral  class).  The  isometric 
HgS  is  known  as  metacinndbarite  and  the  hexagonal  form  as  cinnabar. 
Only  the  latter  is  important.  In  addition  to  these  are  known  the  rare 
compounds  onofrite  (Hg(S-Se)),  tiemannite  (HgSe)  and  color adoite 
(HgTe),  all  of  which  are  isometric. 


98  DESCRIPTIVE  MINERALOGY 

Cinnabar  (HgS) 

Cinnabar  is  the  only  compound  of  mercury  that  occurs  in  sufficient 
quantity  to  constitute  an  important  ore.  Nearly  all  of  the  mercury, 
or  quicksilver,  in  the  world  is  obtained  from  it.  The  mineral  occurs 
both  crystallized  and  massive.  The  ore  is  a  red  crystalline  mass  that 
is  easily  distinguished  from  all  other  red  minerals  by  its  peculiar  shade  of 
color  and  its  great  weight. 

Theoretically,  it  contains  13.8  per  cent  S  and  86.2  per  cent  Hg. 
Massive  cinnabar  is,  however,  usually  impure  through  the  admixture 
of  clay,  iron  oxides  or  bituminous  substances.  Occasionally  the  quan- 
tity of  organic  material  present  is  so  large  that  the  mixture  is  inflam- 
mable. 

Though  cinnabar  is  usually  granular,  massive  or  earthy,  it  some- 
times occurs  beautifully  crystallized 
in  small  complex  and  highly  modi- 
fied hexagonal  crystals  that  exhibit 
tetartohedral  forms  (trigonal  trape- 
zohedral  class).     Usually  the  crys- 
tals are  rhombohedral  or  prismatic 
in    habit.      Their    axial     ratio    is 
1  :  I-I453-      Planes     belonging    to 
more  than   100  distinct  forms  have 
Fig,  38.-CiniK.bar  Crystals  wiih  -  R,    been  observed,  but  the  crystals  on 
10I0  (*);  (R,  404s   (-');  fR,  aoi"s    wnlcn   thcy    occur    are    usually   so 
(ft;  R,  toil  (r)  and  oR,  0001  (.-).        small  that   few  of   them  are  of  im- 
portance as   distinguishing   charac- 
teristics.   The   prismatic   crystals,   which   are   the  most  common  in 
this  country,  are  often  bounded   by   00  R,   (10T0)   and   $R,   (4045) 
(Fig.  38).    Others,  however,  are  very  complicated.    Their  cleavage  is 
perfect  parallel  to  00  R(ioTo). 

The  mineral  is  slightly  sectile.  It  is  transparent,  translucent  or 
opaque,  is  of  a  cochineal-red  color,  often  inclining  to  brown,  and  its 
streak  is  scarlet.  Its  hardness  is  only  2-2.5  and  *ls  density  about 
8.t.  It  is  circularly  polarizing  and  is  a  nonconductor  of  electricity. 
Its  dimorph,  metacinnabarite,  on  the  other  hand,  is  a  good  conductor. 
The  indices  of  refraction  of  cinnabar  are:  01=  2.854,  (=3.201. 

When  heated  gently  in  the  open  tube  cinnabar  yields  sulphurous 
fumes  and  globules  of  mercury.  On  charcoal  before  the  blowpipe  it 
volatilizes  completely. 

There  are  only  a  few  minerals  with  which  cinnabar  is  likely  to  be 


SULPHIDES,  TELLURIDES,   ETC.  99 

confused,  since  its  color  and  streak  are  so  characteristic.  From  all 
red  minerals  but  realgar  it  may  easily  be  distinguished  by  its  sulphur 
reaction.  From  realgar  it  is  distinguished  by  its  great  density  and  its 
greater  hardness. 

Pseudomorphs  of  cinnabar  after  stibnite,  dolomite  ((Ca-Mg)COa), 
pyrite  and  tetrahedrite  (a  complicated  sulpho-salt)  have  been  described. 

Synthesis. — Crystals  have  been  made  by  heating  mercury  in  an  aque- 
ous solution  of  H2S. 

Occurrence,  Localities  and  Origin. — Cinnabar  is  usually  found  in 
veins  cutting  serpentine,  limestones,  slates,  shales  and  various  schists. 
It  is  associated  with  gold,  various  sulphides,  especially  pyrite  and  mar- 
casite  (FeS2),  calcite  (CaCOa),  barite  (BaSCU),  fluorite  (CaF2)  and 
quartz.  It  is  also  found  impregnating  sandstones  and  other  sedimen- 
tary rocks,  and  sometimes  as  a  deposit  from  hot  springs.  Its  deposi- 
tion is  thought  to  be  the  result  of  precipitation  from  ascending  hot 
water. 

Crystallized  cinnabar  occurs  at  a  number  of  places  in  Bohemia, 
Hungary,  Serbia,  Austria,  Spain,  California,  Texas,  Nevada,  and  at 
other  localities  in  Europe,  Asia  and  South  America. 

The  principal  deposits  of  economic  importance  are  at  Almaden 
in  Spain,  at  Idria  in  the  Province  of  Carniola,  Austria,  at  Bakhmut 
in  southern  Russia,  at  various  points  along  the  Coast  Ranges  in  Cal- 
ifornia, in  Esmeralda,  Humboldt,  Nye  and  Washoe  Counties  in  Nevada, 
at  many  points  in  Oregon  and  Utah,  and  at  Terlingua  in  Texas.  The 
mineral  is  also  abundant  in  Peru  and  in  China  but  in  these  countries 
it  has  not  yet  been  mined  profitably.  The  California  cinnabar  district 
extends  for  many  miles  along  the  Coast  Ranges,  but  at  only  about  a 
duzen  places  is  the  mineral  mined. 

The  Spanish  mines,  near  the  city  of  Cordova,  have  been  worked 
for  many  hundreds  of  years.  Much  of  the  ore  is  an  impregnation  of 
sandstone  and  quartzite — the  mineral  sometimes  comprising  as  much 
as  20  per  cent  of  the  rock  mined. 

Extraction. — The  metallurgy  of  cinnabar  is  exceedingly  simple.  It 
consists  simply  in  roasting  the  ore  alone,  or  mixed  with  limestone,  and 
conducting  the  fumes  into  a  condensing  chamber  that  is  kept  cool. 
The  sulphur  gases  are  allowed  to  escape  through  the  chamber  in  which 
the  mercury  is  collected 

Uses  of  Metal. — Mercury  finds  many  uses  in  the  arts.  Its  most  im- 
portant one  is  in  the  extraction  of  gold  and  silver  by  the  amalgamation 
process.  It  is  the  essential  constituent  of  the  pigment  vermilion,  which 
is  a  manufactured  HgS.    In  its  metallic  state  it  is  largely  employed  in 


100  DESCRIPTIVE  MINERALOGY 

the  making  of  mirrors,  of  barometers,  thermometers  and  other  physical 
instruments.  Some  of  the  salts  are  important  medicinal  preparations 
while  others  are  used  in  the  manufacture  of  percussion  caps. 

Production. — The  world's  annual  production  of  quicksilver,  all  of 
which  is  obtained  from  cinnabar,  is  not  far  from  4,000  metric  tons.  The 
United  States  produced  940  tons  in  1912,  valued  at  $1,053,941.  Of  this 
total  California  yielded  20,524  flasks  of  75  lbs.  each,  valued  at  about 
$863,034,  and  Texas  and  Nevada  4,540  flasks  valued  at  $190,907.  To 
produce  these  quantities  of  metal  California  mined  139,347  tons  of  ore 
and  Texas  and  Nevada  16,346  tons.  The  California  ore  yielded  11  lbs. 
of  metal  per  ton  and  the  Nevada  and  Texas  ore  20.8  lbs. 

Metacinnabarite  (HgS)  is  generally  found  as  a  gray-black  massive 
mineral  with  a  black  streak.  It  is  brittle,  has  a  hardness  of  3  and  a 
density  of  7.8.  It  is  associated  with  cinnabar  at  some  of  the  mines  in 
California  and  Mexico,  and  at  a  few  places  in  other  countries.  It  is 
exceedingly  rare. 

THE  METALLIC  DISULPHIDES,  DISELENIDES  AND  DIARSENIDES 

The  disulphides,  diselenides,  ditellurides,  diarsenides  and  dianti- 
monides  differ  from  the  corresponding  monocompounds  in  that  they 
contain  double  the  quantity  of  S,  Se,  Te  and  Sb.  They  are  divisible 
into  two  groups,  one  of  which  comprises  sulphides,  arsenides  and  anti- 
monides  of  iron,  manganese,  cobalt,  nickel  and  platinum,  and  the  other 
the  tellurides  and  selenides  of  gold  and  silver, 

GLANZ  GROUP 

The  glanz  group  is  an  excellent  illustration  of  an  isodimorphous  grbup. 
Its  members  are  characterized  by  their  hardness,  opaqueness,  light  color 
and  brilliant  luster.  Hence  the  name  of  the  group.  In  composition 
the  minerals  belonging  to  the  group  are  sulphides,  arsenides  or  anti- 
monides  of  the  iron-platinum  group  of  metals,  with  the  general  formula 
RQ2  in  which  R  is  Mn,  Fe,  Ni,  Co,  Pt,  and  Q=S,  As  and  Sb.  The  com- 
position of  the  more  simple  members  may  be  represented  by  the  formula 

Fe^  I ,  and  of  those  in  which  arsenic  or  antimony  replaces  a  part  of   the 

XS 

<As=Asv 
\Fe. 
S SX 

:     It  is  probable,  however,  that  some  of  the  cobalt  and  nickel  arsenides 


SULPHIDES,  TELLURIDES,  ETC. 


101 


are  mixtures  and  that  their  indicated  compositions  are  only  approximate. 
All  members  of  the  group  are  believed  to  be  dimorphous,  crystallizing 
in  the  isometric  (dyakisdodecahedral  class),  and  in  the  orthorhombic 
systems  (orthorhombic  bipyramidal  class),  though  not  all  have  as  yet 
been  found  in  both  forms.  The  most  important  members  of  the  group,  as 
at  present  constituted,  are  as  follows: 


Isometric 

Orthorhombic 

Pyrite 

FeS2 

Marcasite 

Haueriie 

MnSs 

FeAsS 

Arsenopyrite 

FeAs2 

Ldllingite 

Cobaltite 

CoAsS 

Glancodot 

Gersdorfite 

(NiFe)AsS 

Korynite 

(Ni-Fe)(As-Sb-S)2 

WolfackUe 

UUmanite 

NiSbS 

Smaltite 

C0AS2 

Safflorite 

CUoanthite 

NiAs2 

Rammelsbergite 

Sperrylite 

PtAS2 

The  group  is  divided  into  two  subgroups,  the  regularly  crystallizing 
minerals  forming  the  pyrite  group  and  the  orthorhombic  ones  the  mar- 
casite group.  The  most  important  members  of  the  former  group  are 
pyrite,  cobaltite,  smalttte  and  chloanthite.  The  most  important  members 
of  the  marcasite  group  are  marcasite,  arsenopyrite  and  loUingite. 

PYRITE   DIVISION 

The  crystallization  of  the  pyrite  group  is  in  the  parallel  hemihedral 
division   (dyakisdodecahedral  class)   of  the   isometric  system.     The 

[20ool 
,  210,  is  so  frequently  seen  on  the  mineral 


occurrence  of  the  form 


pyrite  that  it  has  received  the  name  pyritoid. 

The  group  is  so  perfectly  isomorphous  that  a  description  of  the  forms 
on  one  member  is  practically  a  description  of  the  forms  on  all. 

Pyrite  (FeS2) 

Pyrite,  one  of  the  most  common  of  all  minerals,  is  found  under  a 
great  variety  of  conditions  as  crystals,  as  crystalline  aggregates  and 
as  crystalline  masses.  It  occurs  under  practically  all  conditions  and  in 
all  situations.  It  is  easily  recognized  by  its  bright  yellow  color,  its 
brilliant  luster  and  its  hardness. 


DESCRIPTIVE  MINERALOGY 


Pyrite  containing,  theoretically,  46.6  per  cent  of  iron  and  53.4  per 
cent  of  sulphur  is  usually  contaminated  with  small  quantities  of  nickel, 


Fin.  39. — Group  of  Pyrite  Crystals  in  which  the  Cube  Predominates.     The  i_rystals 
are  striated  parallel  to  the  edge  between  =0  O  =0  (100)  and  l~— —  I ,  i"°)- 


cobalt,  thallium  and  other  elements.     An  auriferous  variety  is  worked 
for  gold,  yielding  in   the  aggregate  a  large  quantity  of  the  precious 


Fig.  40-  Fig.  41. 

FlC.  40.— Pyrite  Crystals  on  which  O  (11 1)  Predominates.     o  =  0,  lit  and  e  =  ™Os 

Fig.  41.— Pyrite  Crystal  with  nOi,  210(c)  andO,  m  (a). 

metal.     Sometimes  arsenic  is  present  in  small  quantity.     Analysis  of 
the  crystals  from  French  Creek,  Penn.,  gave: 


S  =  54.o8,  As=o.20,  Fe=44-24,  Co  =  i.75,  Ni=o.i8,  Cu=o.os,  =100.50. 


SULPHIDES,  TELLURIDES,  ETC.                  103 
The  number  of  forms  that  have  been  observed  on  pyrite  crystals  is 
very  large.    Hintze  records  86.     The  cube  and  the  pyritoid     


Fig.  43. — Group  oi  Pyrite  Crystals  in  which    o»02   (jio)   Predominates.     From 
Daly-Judge  Mine,  neat  Part  City,  Utah.    (After  J.  W.  Boutwell.) 

(210)  arc  the  most  common  of  these,  though  the  octahedron  and  the 

dodecahedron  are  not  rare.     Four  distinct  types  of  crystals  may  be 

recognized,  viz.:  those  with  the  cubic  (Fig.  3g), 

the  octahedral  (Fig.  40),   and   the   pyritoid 

habits  (Figs.  41  and  41),  and  those  that  are 

interpenetrating    twins  (Fig.  43).     The  cubic 

and  the  pyritoid  planes  are   often   striated 

parallel  to  the  edges  between  these  faces.   The 

interpenetrating  twins  are  twinned  about  the 

plane  O(m). 

The  cleavage  of  pyrite  is  imperfect  and 
its  fracture  conchoids!.  The  mineral  is 
brittle.  Its  hardness  is  6-6.5  ar>d  density 
about  5.  Its  luster  is  very  brilliant  and 
metallic.  Its  color  is  brassy  yellow  and  its 
streak  greenish  or  brownish  black.  With  steel  it  strikes  fire,  hence  its 
name  from  the  Greek  word  meaning  fire.  It  is  a  good  conductor  of 
electricity  and  is  strongly  thermo-electric. 


•"ig.  43. — Pyrite  Interpene- 
Iration  Twin.  Two  Pyri- 
tuids  ( °°  O-i,  216) Twinned 
about  O,  in. 


104  DESCRIPTIVE  MINERALOGY 

In  the  closed  tube  pyrite  yields  a  sublimate  of  sulphur  and  a  residue 
that  is  magnetic.  On  charcoal  sulphur  is  freed.  This  burns  with  the 
blue  flame  characteristic  of  the  substance.  The  globule  remaining  after 
heating  for  some  time  is  magnetic.  Treated  with  nitric  acid  the 
mineral  dissolves  leaving  a  flocculent  residue  of  sulphur,  which  when 
dried  and  heated  may  readily  be  ignited. 

Pyrite  in  some  of  its  forms  so  closely  resembles  gold  that  it  is  often 
known  as  fool's  gold.  There  is,  of  course,  no  difficulty  in  distinguishing 
between  the  two  metals,  since  pyrite  contains  sulphur  and  is  soluble  in 
nitric  acid,,  while  gold  contains  no  sulphur  and  is  insoluble  in  all  simple 
acids. 

The  mineral  is  most  easily  confounded  with  chalco pyrite  (CuFeS2), 
though  the  difference  in  hardness  of  the  two  easily  serves  to  distinguish 
them.  Chalcopyrite  may  be  readily  scratched  with  a  knife  blade  or  a 
file,  while  pyrite  resists  both.  The  latter  mineral,  moreover,  contains 
no  copper. 

Syntheses. — Small  crystals  of  pyrite  are  produced  by  the  action 
of  H2S  on  the  oxides  or  the  carbonate  of  iron  enclosed  in  a  sealed  tube 
heated  to  8o°-90°;  also  by  the  passage  of  H2S  and  FeCfe  vapors  through 
a  red-hot  porcelain  tube. 

Occurrence  and  Origin. — Pyrite  occurs  in  veins  and  as  grains  or 
crystals  embedded  in  all  kinds  of  rocks.  In  rocks  it  usually  appears  as 
crystals,  but  in  vein-masses  it  may  appear  either  as  crystals,  with  other 
minerals,  or  as  radiating  or  structureless  masses  occupying  entirely  the 
vein  fissures.  In  slates  it  often  occurs  in  spheroidal  nodules  and 
concretions  of  various  forms,  and  also  as  embedded  crystals.  The 
mineral  is  the  product  of  igneous,  metamorphic  and  aqueous  agencies. 

Pyrite  weathers  readily  to  limonite.  In  ore  bodies  near  the 
surface  it  is  oxidized.  A  portion  of  the  mineral  changes  to  FeS04 
which  percolates  downward  and  aids  in  the  concentration  of  any 
valuable  metals  that  may  be  present  in  small  quantity  in  the  ore. 
Another  portion  of  the  iron  remains  near  the  surface  in  the  form  of 
limonite.  This  covering  of  oxidized  material  is  known  as  the  "  gossan  " 
and  it  is  characteristic  of  all  pyrite  deposits. 

Localities. — Pyrite  crystals  are  so  widely  distributed  that  but  very 
few  of  its  most  important  occurrences  may  be  mentioned  here.  In  the 
mines  of  Cornwall,  Eng.,  and  in  those  on  the  Island  of  Elba  very  large 
crystals  are  found.  Fine  crystals  also  come  from  many  different  places 
in  Bohemia,  Hungary,  Saxony,  Peru,  Norway,  and  Sweden. 

In  the  United  States  the  finest  crystals  are  at  Schoharie  and  Rossie, 
N.  Y.;   at  the  French  Creek  mines  in  Chester  Co.,  and  at  Cornwall, 


SULPHIDES,  TELLURIDES,  ETC.  105 

Lebanon  Co.,  Penn.,  and  near  Greensboro  and  Guilford  Co.,  N.  Carolina. 
Massive  pyrite  occurs  in  great  deposits  at  the  Rio  Tinto  mines  in 
Spain;  at  Rowe,  Mass.;  in  St.  Lawrence  and  Ulster  counties,  N.  Y.; 
in  Louise  Co.,  Va.,  and  in  Paulding  Co.,  Ga.  Much  of  the  massive 
pyrite  in  the  veins  of  Colorado,  California  and  of  the  southern  states, 
from  Virginia  to  Alabama,  is  auriferous  and  much  of  it  is  mined  for  the 
gold  it  contains. 

Uses. — Pyrite  is  used  principally  in  the  manufacture  of  sulphuric 
acid.  The  mineral  is  burned  in  furnaces  and  the  SO2  gases  thus  result- 
ing are  carried  to  condensers  where  they  are  oxidized  by  finely  divided 
platinum  or  by  the  oxides  of  nitrogen.  The  residue,  which  consists 
largely  of  Fe203,  is  sometimes  smelted  for  iron  or  made  into  paint. 
This  residue  also  contains  the  gold  and  other  valuable  metals  that  may 
have  been  in  the  original  pyrite. 

The  sulphuric  acid  obtained  from  pyrite  enters  into  many  manu- 
facturing processes.  The  greater  portion  of  it  is  consumed  in  the 
artificial  fertilizer  industry. 

Production. — Pyrite  is  mined  in  the  United  States  in  Franklin  Co., 
Mass.,  in  Alameda  and  Shasta  Counties,  California,  in  Louisa,  Pulaski 
and  Prince  William  Counties,  Va.,  in  Carroll  Co.,  Ga.,  in  St.  Lawrence 
Co.,  N.  Y.,  in  Clay  Co.,  Alabama,  and  at  the  coal  mines  in  Ohio, 
Illinois  and  Indiana  where  it  is  a  by-product.  The  total  production 
of  the  United  States  in  191 2,  amounting  to  350,928  long  tons,  was 
valued  at  $1,334,259.  Virginia  is  by  far  the  largest  producer.  In 
addition  to  this  quantity  the  trade  consumed  970,785  tons  of  imported 
ore,  most  of  which  came  from  Spain,  and  utilized  the  equivalent  of 
260,000  tons  of  pyrite  in  the  shape  of  low  grade  sulphide  copper  ores 
from  Ducktown,  Tenn.,  and  zinc  sulphide  concentrates  from  the  Mis- 
sissippi Valley  and  elsewhere  for  the  manufacture  of  sulphuric  acid. 
The  total  amount  of  sulphuric  acid  manufactured  in  the  United  States 
during  191 2  was  2,340,000  short  tons,  valued  at  $18,338,019.  The  total 
world  production  of  pyrite  is  about  2,000,000  tons  annually. 

Small  quantities  of  the  mineral  are  also  mined  for  local  consumption 
in  Lumpkin  Co.,  Georgia,  and  near  Hot  Springs,  Arkansas.  Much 
auriferous  pyrite  has  also  been  mined  in  the  southern  states  and  the 
Rocky  Mountain  region  for  the  gold  it  contains.  This  metal  is  sepa- 
rated from  the  pyrite  partly  by  crushing  and  amalgamation  and  partly 
by  smelting  or  by  leaching  processes.  In  the  former  case  the  gold 
occurs  as  inclusions  of  the  metal  in  the  pyrite. 


106  DESCRIPTIVE  MINERALOGY 


Cobaltite  (CoAsS) 

Cobaltite  is  a  silver-white  or  steel-gray  mineral  occurring  in  massive 
forms  or  in  distinct  crystals  exhibiting  beautifully  their  hemihedral 
character.  It  is  completely  isomorphous  with  the  corresponding  nickel 
compound,  gersdorffite  (NiAsS),  and  consequently  mixtures  of  the 
two  are  common. 

Cobaltite  usually  contains  some  iron  and  often  a  little  nickel. 
Theoretically,  it  consists  of  19.3  per  cent  S,  45.2  per  cent  As  and  35.5 
Co.  The  compositions  of  a  massive  variety  from  Siegen,  Westphalia, 
and  that  of  crystals  from  Nordmark,  Norway,  are  as  follows: 

As  S  Co  Fe  Ni  Total 

Siegen 45.31         19.35        33-71         163  100.00 

Nordmark 4477        20.23        29.17        4.72        1.68        100.57 

The  crystallization  of  cobaltite  is  perfectly  isomorphous  with  that 
of  pyrite,  though  the  number  of  its  forms  observed  is  far  smaller.    The 

[2O  00  1 
(210). 

The  cleavage  of  cobalt  is  fairly  good  parallel  to  00  O  00  (100).  Its 
fracture  is  uneven,  its  hardness  is  5.5  and  its  density  about  6.2.  The  color 
of  the  mineral,  as  stated  above,  varies  between  silver-white  and  steel- 
gray.     Its  streak  is  grayish  black.     It  is  a  good  conductor  of  electricity. 

In  the  open  tube  cobaltite  reacts  for  S  and  As.  On  charcoal  it 
yields  a  magnetic  globule  which  when  fused  with  borax  on  platinum 
wire  yields  a  deep  blue  bead.  It  weathers  fairly  readily  to  the  rose- 
colored  cobalt  arsenate  known  as  erythrite  (Co3(As04)2*8H20). 

By  its  crystallization  and  color  cobaltite  is  distinguished  from 
nearly  all  other  minerals  but  those  of  the  same  group.  From  most  of 
these  it  is  easily  distinguished  by  its  blowpipe  reactions  for  sulphur, 
arsenic  and  cobalt. 

Occurrence  and  Origin. — Cobaltite  occurs  mainly  in  veins  that  are 
believed  to  have  been  filled  by  upward  moving  solutions  emanating 
from  igneous  rocks.  It  is  associated  with  compounds  of  nickel  and  other 
cobalt  compounds  and  with  silver  and  copper  ores. 

Localities. — Cobaltite  is  not  very  widely  distributed.  Large,  hand- 
some crystals  occur  at  Tunaberg  in  Sweden;  at  Nordmark,  Norway; 
at  Siegen,  Westphalia,  and  near  St.  Just  in  Cornwall,  England.  It  is 
found  also  in  large  quantity  at  Cobalt,  Ontario,  associated  with  silver 
ores  and  nickel  compounds. 


SULPHIDES,  TELLURIDES,  ETC.  107 

Uses. — Cobaltite  is  said  to  be  used  by  jewelers  in  India  in  the  pro- 
duction of  a  blue  enamel  on  gold  ornaments.  It  is  employed  also  in  the 
manufacture  of  blue  and  green  pigments  and  in  the  manufacture  of  com- 
pounds used  in  small  quantity  in  the  various  arts.  Smalt  is  the  most 
valuable  of  the  cobalt  pigments  and  is  at  present  the  chief  commercial 
compound  of  this  metal.  It  is  a  deep  blue  glass  that  differs  from 
ordinary  glass  in  containing  cobalt  in  place  of  calcium.  Smalt  is  hiade 
from  cobaltite  and  from  other  cobalt  ores  by  fusion  with  a  mixture  of 
quartz  and  potassium  carbonate.  Certain  cobalt  compounds  are  sug- 
gested as  excellent  driers  for  oils  and  varnishes.  The  mineral  is  also 
utilized  as  an  ore  of  cobalt,  which  in  the  form  of  stellite,  an  alloy  com- 
posed of  70  per  cent  cobalt,  15  per  cent  chromium  and  15  per  cent 
molybdenum  or  tungsten,  bids  fair  to  acquire  a  large  use  as  a  material 
for  the  manufacture  of  table  cutlery  and  edged  tools.  The  use  of  the 
metal  has  also  been  suggested  as  a  material  for  coinage  in  place  of 
nickel. 

Production. — Most  of  the  cobalt  of  commerce  is  handled  by  the 
trade  in  the  form  of  the  oxide.  It  is  produced  from  the  various  cobalt 
minerals,  mainly  as  a  by-product  in  the  extraction  of  nickel,  and  hence 
very  little  is  obtained  from  cobaltite.  The  mines  at  Cobalt,  however, 
have  furnished  a  large  quantity  of  cobaltite  and  smaltite  within  the  past 
few  years  and  these  have  gone  into  the  manufacture  of  the  oxide,  of 
which  about  515  tons  were  produced  in  191 2,  having  a  value  of 
$317*165. 

Smaltite  (CoAs2) 

Smaltite  is  another  important  ore  of  cobalt.  It  is  found  in  crystals 
and  masses. 

Its  theoretical  composition  is  71.88  per  cent  As  and  28.12  per  cent 
Co,  though  it  usually  contains  also  S,  Ni,  Fe  and  frequently  traces  of 
Bi,  Cu  and  Pb.  Since  it  is  isomorphous  with  the  arsenide  of  nickel 
chloanthite  (N1AS2),  mixed  crystals  of  the  two  are  common.  Moreover, 
sharply  defined  crystals  have  been  found  to  consist  of  mechanical  mix- 
tures of  several  compounds. 

Smaltite  occurs  in  small  crystals  of  cubical  habit  with  00  O  00  (100), 
O(in)  and  various  pyritoids  predominating. 

The  mineral  is  tin-white  to  steel-gray,  and  opaque,  and  has  a  grayish 
black  streak.  It  is  often  covered  with  an  iridescent  or  a  gray  tarnish. 
Its  cleavage  is  indistinct,  its  fracture  uneven,  its  hardness  5-6  and 
density  6.3-7.     It  is  a  good  electrical  conductor. 

Before  the  blowpipe  on  charcoal  smaltite  yields  arsenic  fumes  and  a 


108  DESCRIPTIVE  MINERALOGY 

magnetic  globule  of  metallic  cobalt.  It  is  soluble  in  HNO3,  yielding  a 
rose-colored  solution  and  a  precipitate  of  AS2O3. 

The  mineral  is  fairly  easily  distinguished  from  most  other  minerals 
by  its  color  and  blowpipe  reactions.  From  cobaltite  it  is  distinguished  by 
the  lack  of  S.  From  a  few  others  that  are  not  described  in  this  volume 
it  can  be  distinguished  by  its  crystallization  or  by  quantitative  analysis. 

Synthesis. — Smaltite  crystals  are  produced  when  hydrogen  acts  at  a 
high  temperature  upon  a  mixture  of  the  chlorides  of  cobalt  and  arsenic. 

Occurrence  and  Origin. — Smaltite  is  found  associated  with  cobaltite 
in  nearly  all  of  its  occurrences.  It  is  especially  abundant  at  Cobalt,  Ont. 
As  in  the  case  of  most  other  cobalt  minerals,  its  presence  is  indicated  by 
deposits  of  rose-colored  erythrite  which  coat  its  surfaces  wherever  these 
are  exposed  to  moist  air.  Its  methods  of  occurrence,  origin  and  uses 
are  the  same  as  for  cobaltite  (p.  107). 

Chloanthite  (NiAs2)  resembles  smaltite  in  most  of  its  characteris- 
tics. The  two  minerals  grade  into  each  other  through  isomorphous 
mixtures.  Those  mixtures  in  which  the  cobalt  arsenide  is  in  excess 
are  known  as  smaltite,  while  those  in  which  NiAs  predominates  are 
called  chloanthite.  The  pure  chloanthite  molecule  is  Ni=  28.1  per  cent, 
As  =71.9  per  cent. 

The  two  minerals  can  be  distinguished  when  unmixed  with  one 
another  by  the  blowpipe  reactions  for  Co  and  Ni.  In  mixed  crystals 
the  predominance  of  one  or  the  other  arsenides  can  be  determined  only 
by  quantitative  analysis. 

Chloanthite  containing  much  iron  is  distinguished  as  chathamite, 
from  Chatham,  Conn.,  where  it  occurs  with  arsenopyrite  and  niccolite  in 
a  mica-slate. 

The  mode  of  occurrence  of  chloanthite  and  the  localities  at  which 
it  is  found  are  the  same  as  in  the  case  of  smaltite. 

Sperrylite  (PtAs2) 

Sperrylite  is  extremely  rare.  It  is  referred  to  here  because  it  is  the 
only  platinum  compound  occurring  as  a  mineral.  Chemically,  it  is 
43.53  per  cent  As  and  56.47  per  cent  Pt,  but  it  contains  also  small  quan- 
tities of  Sb,  Pd  and  Fe. 

Its  crystals  are  simple.  They  contain  only  O(in),  00  O  00  (100), 
00  0(i  10)  and  several  pyritoids.  Their  habit  is  usually  octahedral  or 
cubical. 

The  mineral  is  opaque  and  tin-white,  and  its  streak  black.  Its  hard- 
ness is  6-7  and  density  10.6. 


SULPHIDES,  TELLURIDES,  ETC.  109 

In  the  closed  glass  tube  it  remains  unchanged,  but  in  the  open  tube 
it  gives  a  sublimate  of  AS2O3.  When  dropped  upon  red-hot  platinum 
foil  it  immediately  melts,  giving  rise  to  fumes  of  AS2O3,  and  forming 
blisters  on  the  foil  that  are  not  distinguishable  from  the  original  platinum 
in  color  or  general  character.  It  is  slowly  soluble  in  concentrated  HC1 
and  aqua  regia. 

Synthesis. — The  mineral  has  been  produced  by  leading  arsenic  fumes 
over  red-hot  platinum  in  an  atmosphere  of  hydrogen. 

Occurrence  and  Localities. — Sperrylite  occurs  as  little  crystals  com- 
pletely embedded  in  the  chalcopyrite  (CuFeS2)  and  the  gossan  of  a 
nickel  mine,  and  in  the  chalcopyrite  of  a  gold-quartz  vein  near  Sudbury, 
Ontario;  in  covellite  at  the  Rambler  Mine,  Encampment,  Wyoming; 
and  as  flakes  in  the  sands  of  streams  in  the  Cowee  Valley,  Macon  Co.,  Ga. 
The  flakes  resemble  very  closely  native  platinum,  from  which  they  are 
of  course,  easily  distinguished  by  the  test  for  arsenic. 

Uses. — The  sperrylite  from  Sudbury  and  Wyoming  furnish  much  of 
the  platinum  produced  in  the  United  States  (see  p.  64). 

MARCASITE   DIVISION 

Three  members  of  the  marcasite  group  are  important;  all  are  inter- 
esting from  the  fact  that  they  are  so  alike  in  their  crystallization  that  a 
description  of  the  forms  belonging  to  any  one  of  them  might  serve  as  a 
description  of  those  belonging  to  all  others.  The  crystallization  of  the 
group  is  orthorhombic  (rhombic  bipyramidal  class),  with  an  axial  ratio 
approximately  a  :  b  :  c=.y  :  1  :  1.2. 

m 

Marcasite  (FeS2) 

Marcasite,  the  dimorph  of  pyrite,  resembles  this  mineral  so  dosely 
that  in  massive  specimens  it  is  difficult  to  distinguish  between  the  two. 
They  are  nearly  alike  in  hardness,  in  color  and  in  chemical  properties. 
Marcasite  is  a  little  lighter  in  color  than  pyrite.  Its  density  is  less 
(about  4.9),  and  it  possesses  a  greater  tendency  to  tarnish  on  exposed 
surfaces. 

This  tarnish  indicates  that  the  mineral  is  more  susceptible  to  altera- 
tion than  is  pyrite.  One  of  the  products  of  this  alteration  is  ferrous  sul- 
phate, which  may  often  be  detected  by  its  taste  upon  touching  the  tongue 
to  specimens  of  the  mineral.  In  crystallized  specimens  there  is  not  the 
least  difficulty  in  distinguishing  between  them,  since  their  crystallization 
is  very  different. 

Marcasite  is  orthorhombic  (rhombic  bipyramidal  class),  with  the 


110  DESCRIPTIVE  MINERALOGY 

axial  ratio  .7662  :  1  :  1.2342.  Its  simple  crystals  often  possess  a  tabular 
or  a  pyramidal  habit  (Figs.  44  and  45).  In  the  former  case  oP(ooi)  is 
the  predominant  face,  and  in  the  latter  case  the  two  domes  P  66  (101) 


Fie.  45. 


Fie.  44.— Mucuite  Crystal  with  «P,no(m);  oP,  001  (e);  PS,  ,011  (0  and  JPB  , 

013  (»). 

Fig.  45. — Marcasite  Crystal  with  Forms  as  Indicated  in  Fig.  44,  and  P  » ,  101  («) 

andP,  in  (s). 

and  P  06  (on).    The  other  forms  observed  on  most  crystals  are  00  P(no), 
P(iii),  and  often  jP  *  (013). 

Twins  are  very  common,  with  00  P(i  10)  the  twinning  plane  (Fig.  46). 
Sometimes  these  are  aggregated  by  repeated  twinning  into  serrated 
groups  known  as  cockscomb  twins  or  spearhead  twins  (Fig.  47),  because 


Fie.  46. 

Fig.  46. — Twin  of  Marcasite  about  » 


of  the  outlines  of  their  edges.  In  many  instances  the  crystals  are  acic- 
ular  or  columnar  in  habit,  forming  radiating  groups  with  globular,  reni- 
form  and  stalactitk  shapes.  Concretions  are  also  common.  The  basal 
plane  is  usually  striated  parallel  to  the  edge  between  it  and  P06  (on). 
The  cleavage  is  distinct  parallel  to  00  P(no).     The  fracture  is  uneven 


SULPHIDES,  TELLURIDES,  ETC.  Ill 

When  powdered  marcasite  is  treated  with  cold  nitric  acid  and 
allowed  to  stand,  it  decomposes  with  the  separation  of  sulphur. 

Marcasite  readily  alters  to  limonite.  The  fact  that  pyrite,  sphaler- 
ite, chalcopyrite,  and  other  minerals  form  pseudomorphs  after  it 
indicates  that,  under  suitable  conditions,  it  alters  also  to  these  com- 
pounds. The  mineral  is  in  most  cases  a  direct  result  of  precipitation 
from  hot  solutions. 

Synthesis. — Marcasite  crystals  have  been  prepared  by  the  reduction 
of  FeS04  by  charcoal  in  an  atmosphere  of  HaS. 

Occurrence  and  Uses. — The  mineral,  like  pyrite,  is  found  embedded 
in  rocks  in  the  form  of  crystals  and  concretions,  and  also  as  the 
gangue  masses  of  veins.  It  constitutes  nearly  the  entire  filling  of  some 
veins,  and  forms  druses  on  the  walls  of  cavities  in  both  rocks  and  miner- 
als. It  also  replaces  the  organic  matter  of  fossils  preserving  their  shapes 
— thus  producing  true  pseudomorphs. 

When  associated  with  pyrite  it  is  mined  together  with  this  mineral 
as  a  source  of  sulphur. 

Localities. — Crystalline  marcasite  occurs  in  such  great  quantity 
near  Carlsbad  in  Bohemia  that  it  is  mined.  The  cockscomb  variety  is 
found  in  Derbyshire,  England,  and  crystals  at  Schemnitz  in  Hungary 
and  at  Andreasberg  and  other  places  in  the  Harz.  In  the  United  States 
the  mineral  occurs  as  crystals  at  a  great  number  of  places,  being  par- 
ticularly abundant  in  the  lead  and  zinc  localities  of  the  Mississippi 
Valley,  where  it  sometimes  forms  stalactites.  The  stalactites  from 
Galena,  111.,  often  consist  of  concentric  layers  of  sphalerite,  galena  and 
crystallized  marcasite. 

Arsenopyrite  (FeAsS) 

Arsenopyrite,  or  mispickel,  is  the  most  important  ore  of  arsenic. 
It  is  found  in  crystals  and  in  compact  ar.d  granul&r  masses.  It  is  a 
silver-white  metallic  mineral  resembling  very  closely  cobaltite  in  its 
general  appearance. 

The  formula  FeAsS  for  arsenopyrite  is  based  on  analyses  like  the 
following: 

As  S  Fe       Total 

Specimen  from  Hohenstein,  Saxony ....  45 .  62    19 .  76    34 .  64    100 .  02 
Specimen  from  Mte.  Chalanches,  France  45 .78    19 .  56    34 .64      99 .  98 

Theoretically,  the  mineral  consists  of  its  components  in  the  following 
proportions,  As  46  per  cent,  S  19.7  per  cent,  Fe  34.3  per  cent.  In  many 
specimens  the  iron  is  replaced  in  part  by  cobalt,  nickel  or  manganese. 


112 


DESCRIPTIVE  MINERALOGY 


lw    m  m        I 


Fig.  48. — Arsenopyrite  Crystals    with    00  P, 
no  (m);  JP  00  ,  014  (m),  and  P  00  ,  on  (q). 


Sometimes  the  cobalt  is  present  in  such  large  quantity  that  the  mineral 
is  smelted  as  an  ore  of  this  metal. 

The  axial  ratio  of  arsenopyrite  is  .6773  :  *  :  1-1882.  Its  crystals  are 
usually  simpler  than  those  of  marcasite  (Fig.  48),  though  the  number  of 
planes  observed  in  the  species  is  larger.    Most  of  the  untwinned  crystals 

are  a  combination  of  00  P(no) 
with  £P  06  (014),  or  P  06  (on), 
or  Px>(ioi),  and  have  a  pris- 
matic habit.  Twins  are  not 
rare.  The  twinning  plane  is 
the  same  as  in  marcasite, 
and  repetition  is  often  met 
with.  The  angle  iioai^o= 
68°  13'. 

The  brachydomes  are  stri- 
ated  horizontally,  and    often 
the  planes  ooP(no)  are  stri- 
ated parallel  to  the  edge  00  P(iio)aP*  (ioi). 

The  cleavage  of  arsenopyrite  is  quite  perfect  parallel  to  ooP(no). 
The  mineral  is  brittle  and  its  fracture  uneven.  Its  hardness  is  5.5-6 
and  density  about  6.2.  Its  color  is  silver- white  to  steel-gray;  its  streak 
grayish  black.     It  is  a  good  conductor  of  electricity. 

In  the  closed  tube  arsenopyrite  at  first  gives  a  red  sublimate  of  AsS 
and  then  a  black  mirror  of  arsenic.  On  charcoal  it  gives  the  usual 
reactions  for  sulphur  and  arsenic.  Cobaltiferous  varieties  react  for 
cobalt  with  borax.  The  mineral  yields  sparks  when  struck  with  steel 
and  emits  an  arsenic  smell.  It  dissolves  in  nitric  acid  with  the  separa- 
tion  of  sulphur. 

Arsenopyrite  is  distinguished  from  the  cobalt  sulphides  and  arsenides 
by  the  absence  of  Co. 

Synthesis. — Crystals  of  the  mineral  are  produced  by  heating  in  a 
closed  tube  at  3000  precipitated  FeAsS  in  a  solution  of  NaHCC>3. 

Occurrence. — Arsenopyrite  crystals  are  often  found  disseminated 
through  crystalline  rocks,  and  often  embedded  in  the  gangue  minerals  of 
veins.  Like  pyrite  and  marcasite  they  frequently  fill  vein  fissures.  Its 
associates  are  silver,  tin  and  lead  ores,  chalcopyrite,  pyrite  and  sphalerite. 
Localities. — The  mineral  is  abundant  at  Freiberg,  in  Saxony,  at 
Tunaberg,  in  Sweden,  and  at  Inquisivi  Mt.,  Sorato,  in  Bolivia. 

It  also  occurs  in  fine  crystals  at  Franconia  in  New  Hampshire,  at 
Blue  Hill  in  Maine,  at  Chatham  in  Connecticut,  and  at  St.  Francois, 
Beauce  Co.,  Quebec.     Massive  arsenopyrite  is  found  near  Keeseville 


SULPHIDES,  TELLURIDES,  ETC.  113 

Essex  Co.,  near  Edenville,  Orange  Co.,  and  near  Carmel,  Putnam  Co., 
N.  Y.,  and  at  Rewald,  Floyd  Co.,  Va.  In  most  cases  it  is  apparently 
a  result  of  pneumatolysis. 

Uses. — Arsenopyrite  was  formerly  the  source  of  nearly  all  the  arsenic 
of  commerce.  The  mineral  is  concentrated  by  mechanical  methods,  and 
the  concentrates  are  heated  in  retorts,  when  the  following  reaction  takes 
place:  FeAsS  =  FeS+As.  The  arsenic  being  volatile  is  conducted 
into  condensing  chambers  where  it  is  collected.  When  the  mineral  con- 
tains a  reasonable  amount  of  cobalt  or  of  gold  these  metals  are  extracted. 

Uses  of  Arsenic. — The  metal  arsenic  has  very  little  use  in  the  arts, 
though  its  compounds  find  many  applications  as  insecticides,  medicines, 
pigments,  in  tanning,  etc.  The  basis  of  most  of  these  is  AS2O3,  and 
this  is  produced  directly  from  the  fumes  of  smelters  working  on  arsenical 
gold,  silver  and  copper  ores.  Only  a  portion  of  such  fumes  are  saved, 
however,  as  even  half  of  those  produced  at  a  single  smelter  center 
(Butte,  Montana),  would  more  than  supply  the  entire  demand  of  the 
United  States  for  arsenic  and  its  compounds.  Under  these  conditions 
the  mining  of  arsenical  pyrite  as  a  source  of  arsenic  has  ceased  so  far 
as  the  United  States  is  concerned. 

Lollingite  (FeAso)  is  usually  massive,  though  its  rare  crystals  are 
isomorphous  in  every  respect  with  those  of  arsenopyrite.  The  pure 
mineral  is  not  common.  Most  specimens  are  mixtures  of  ldllingite  with 
arsenopyrite  or  other  sulphides  or  arsenides. 

The  mineral  is  silver-white  or  steel-gray.  Its  streak  is  grayish  black. 
Its  hardness  is  5-5.5  and  density  about  7.2.  It  readily  fuses  to  a  mag- 
netic globule,  at  the  same  time  evolving  arsenic  fumes.     It  is  soluble  in 

HNO3. 

It  usually  occurs  in  veins  associated  with  other  sulphides  and  arsen- 
ides. It  is  found  at  Paris,  Maine;  at  Edenville  and  Monroe,  N.  Y.; 
at  various  mines  in  North  Carolina,  and  on  Brush  Creek,  Gunnison 
Co.,  Colo.  At  the  last-named  locality  the  mineral  is  in  star-shaped 
crystalline  aggregates,  in  twins  and  trillings,  associated  with  siderite 
and  barite. 

SYLVANITE  GROUP 

The  sylvanite  group  includes  at  least  three  distinct  minerals,  all  of 
which  are  ditellurides  of  gold  or  silver.  The  group  is  isodimorphous. 
The  pure  gold  telluride  is  known  only  in  monoclinic  crystals,  but  the 
isomorphous  mixtures  of  the  gold  and  silver  compounds  occur  both  in 
monoclinic  and  orthorhombic  crystals. 


114  DESCRIPTIVE  MINERALOGY 

i 

Orthorhombic  bipyramidal  Monoclinic  prismatic 

AuTe2  Calaverite 

Krenneriie  (Ag  •  Au)Te2  SylvaniU 

All  three  minerals  are  utilized  as  ores  of  gold.  While  occurring  only 
in  a  few  places,  they  are  sufficiently  abundant  at  some  to  be  mined. 

Calaverite  (AuTe2) 

Calaverite  is  a  nearly  pure  gold  chloride.  However,  it  is  usually 
intermixed  with  small  quantities  of  the  silver  telluride.  An  analysis  of  a 
specimen  from  Kalgoorlie,  Australia,  gave:  Te  =  57.27;  Au  =  4i.37; 
Ag=.s8. 

Calaverite  crystallizes  in  the  monoclinic  system  (prismatic  class)  in 
crystals  that  are  elongated  parallel  to  the  orthoaxis  and  deeply  striated 
in  this  direction.     Their  axial  ratio  is  1.6313  :  1  :  1.1440  with  £=90°  13'. 

The  prominent  forms  are  00  P  06  (100),  00  P  00  (010),  oP(ooi), 
— P*(ioi),  +Pob(ioT),  —  2P«x)(20i),  +2P«>(2oT),  and  P(in). 
Twinning  is  common  and  the  resulting  twins  are  very  complicated. 
Usually,  however,  the  mineral  occurs  massive  and  granular. 

Calaverite  is  opaque,  silver-white  or  bronzy  yellow  in  color  and  has  a 
yellow-gray  or  greenish  gray  streak.  Its  surface  is  frequently  covered 
with  a  yellow  tarnish.  The  mineral  is  brittle  and  without  distinct  cleav- 
age.   Its  hardness  is  2-3  and  density  9.04. 

On  charcoal  before  the  blowpipe  the  mineral  fuses  easily  to  a  yellow 
globule  of  gold,  yielding  at  the  same  time  the  fumes  of  tellurium  oxide. 
It  dissolves  in  concentrated  H2SO4,  producing  a  deep  red  solution.  When 
treated  with  HNO3  it  decomposes,  leaving  a  rusty  mass  of  spongy  gold. 
The  solution  treated  with  HC1  usually  yields  a  slight  precipitate  of  silver 
chloride. 

Calaverite  is  distinguished  from  most  other  minerals  by  the  test  for 
tellurium.  It  is  distinguished  from  fetzite  (p.  80),  by  its  crystallization 
and  the  fact  that  it  gives  a  yellow  globule  when  roasted  on  charcoal, 
and  from  sylvanite  by  the  small  amount  of  silver  it  contains,  its  higher 
specific  gravity,  its  color  and  its  lack  of  cleavage.  It  is  distinguished 
from  krenneriie  by  its  crystallization. 

Occurrence. — The  mineral  occurs  in  veins  with  the  other  tellurides 
associated  with  gold  ores  in  Calaveras  Co.,  Cal.,  and  at  the  localities 
mentioned  for  petzite  (see  p.  81).  It  is  believed  to  have  been  deposited 
by  pneumatolytic  processes  or  by  ascending  magmatic  water  at  com- 
paratively low  temperatures. 


SULPHIDES,  TELLURIDES,  ETC.  115 

Uses. — The  mineral  is  mined  with  other  tellurides  in  Boulder  Co., 
and  at  Cripple  Creek,  Colorado,  as  an  ore  of  gold. 

Sylvanite  (Ag-Au)Te2 

Sylvanite  is  more  common  than  calaverite.  It  is  an  isomorphous 
mixture  of  gold  and  silver  tellurides  in  the  ratio  of  about  i  :  i.  Analyses 
follow: 

I.   Te=62.i6  Au=24.45  Ag—13.39         Total=ioo.oo 

II.    Te=59.78  Au=26.36  Ag=i3.86  "        100.00 

III.    Te=58.9i  Au=29«35  Ag=ii-74  "        100.00 

I." Theoretical  for  AgTea+AuTe,. 
II.  and  III.  Specimens  from  Boulder  Co.,  Colo. 

In  crystallization  the  mineral  is  isomorphous  with  calaverite,  with 
an  axial  ratio  a  :  b  :  c— 1.6339  :  1  :  1.1265  and  0=90°  25'.  Its  crystals 
are  usually  rich  in  planes,  about  75  having  been  identified.  Their  habit 
is  usually  tabular  parallel  to  00  P  ob  (010),  with  this  plane,  —  P  «>  (101), 
oP(ooi),  00  P  6b  (100)  and  2P2(?2i)  predominating.  The  mineral  also 
occurs  in  skeleton  crystals  and  in  aggregates  that  are  piaty  or  granular. 
Twinning  is  common,  with  —  P*(ioi)  the  twinning  plane.  Many 
twinned  aggregates  form  networks  suggesting  writing,  hence  the  name 
"  Schrifterz  "  often  applied  to  the  mineral  by  the  Germans. 

Sylvanite  is  silver-white  or  steel-gray  and  has  a  brilliant  metallic 
luster  and  a  silver-white  or  yellowish  gray  streak.  Its  hardness  is 
between  1  and  2  and  its  density  7.9-8.3.  Moreover,  it  possesses  a  per- 
fect cleavage  parallel  to  ooPw  (010). 

Its  chemical  properties  are  the  same  as  those  of  calaverite,  but  the 
silver  precipitate  produced  by  adding  HC1  to  its  solution  in  HNO3  is 
always  large.  It  is  best  distinguished  from  the  gold  telluride  by  its 
cleavage  and  from  fetzite  ((Ag.Au)2Te)  and  hessite  (Ag2Te)  by  its 
crystallization,  and  by  the  yellow  metallic  globule  produced  when  the 
mineral  is  roasted  on  charcoal.  It  is  distinguishable  from  krennerite  by 
its  crystallization. 

Localities  and  Origin, — Sylvanite  occurs  with  the  other  tellurides  in 
veins  at  Offenbdnya  and  Nagyag  in  Transylvania,  at  Cripple  Creek  and 
in  Boulder  Co.,  Colo.,  near  Kalgoorlie,  W.  Australia,  in  small  quan- 
tities near  Balmoral  in  the  Black  Hills,  S.D.,  and  at  Moss, near  Thunder 
Bay,  Ontario.  Like  calaverite  it  was  deposited  by  magmatic  water,  or 
by  hot  vapors. 

Uses. — It  is  mined  with  calaverite  as  a  gold  and  silver  ore  at  Cripple 
Creek  and  in  Boulder  Co.,  Colo. 


CHAPTER  V 
THE  SULPHO-SALTS  AND   SULPHO-FERRITES 

The  sulpho-salts  are  salts  of  acids  analogous  to  arsenic  acid,  H3ASO4, 
and  arsenious  acid,  H3ASO3,  and  the  corresponding  antimony  acids 
H3SDO4  and  H3SDO3.  The  sulpho-acids  differ  from  the  arsenic  and  the 
antimony  acids  in  containing  sulphur  in  place  of  oxygen,  thus  H3ASS4, 
H3ASS3,  H3SDS4  and  H3SDS3.  The  mineral  enargite  may  be  regarded  as 
a  salt  of  sulpharsenic  acid,  thus  C113ASS4,  copper  having  replaced  the 
hydrogen  of  the  acid.  Proustite,  on  the  other  hand,  is  AgaAsS3,  or  a, 
salt  of  sulpharsenious  acid.  The  salts  of  sulpharsenic  acid  are  called 
sulpharsenates,  while  those  derived  from  sulpharsenious  acid  are  known 
as  sulpharsenites.  The  sulpharsenates  are  not  represented  among  the 
commoner  minerals,  although  the  copper  salt  enargite  is  abundant  at  a 
few  places.  A  number  of  salts  of  other  sulphur-arsenic  acids  are  known 
but  they  are  comparatively  rare. 

There  is  another  class  of  compounds  with  compositions  analogous 
to  those  of  the  sulpho-salts,  though  their  chemical  nature  is  not  well 
understood.  This  is  the  group  of  the  sulpho-ferrites.  We  know  that 
certain  hydroxides  of  iron  may  act  as  acids  under  certain  conditions. 
The  sulpho-ferrites  may  be  looked  upon  as  salts  of  these  acids  in  which, 
however,  the  oxygen  has  been  replaced  by  sulphur,  as  in  the  case  of  the 
sulpho-acids  referred  to  above.  Thus  by  replacement  of  O  by  S,  in 
ferric  hydroxide  Fe(OH)3  the  compound  Fe(SH;3  or  IfcFeSs  results. 
The  salts  of  this  acid  are  sulpho-ferrites.  This  acid,  by  loss  of  H2S, 
may  give  rise  to  other  acids  in  the  same  way  that  sulphuric  acid  (H2SO4), 
by  loss  of  H2O,  gives  rise  to  pyrosulphuric  acid.  In  the  case  of  the 
sulpho-acid  we  may  have  H3FeS3— H2S  =  HFeS2.  The  copper  salt  of 
this  acid  is  the  common  mineral  chalcopyrite,  CuFeS2- 

The  sulpho-salts  are  very  numerous,  but  only  a  few  of  them  are  of 
sufficient  importance  to  warrant  a  description  in  this  book. 


116 


SULPHO-SALTS  AND  SULPHO-FERRITES  117 


THE  SULPHARSENITES  AND  SULPHANTTMONITES 

The  sulpharsenites  and  sulphantimonites  are  derivatives  of  the 
ortho  acids  H3ASS3  and  H3SbS3. 


ORTHO  SULPHO-SALTS 

The  ortho  salts  are  compounds  in  which  the  hydrogen  of  the  ortho 
acids  is  replaced  by  metals.  They  include  a  large  number  of  minerals, 
of  which  the  following  are  the  most  important. 

Bournonite    (Cu2  ■  Pb)3  (SbSs)2  Orthorhombic 

Pyrargyrite  Ag3SbS3  Hexagonal 

ProuslUe       Ag3AsS3  Hexagonal 

PYRARGYRITE  GROUP 

Pyrargyrite  (Ag3SbSs) 

Pyrargyrite,  or  dark  ruby  silver,  is  an  important  silver  ore,  especially 
in  Mexico,  Chile  and  the  western  United  States.  The  name  ruby  silver 
is  given  to  it  because  thin  splinters  transmit  deep  red  light.  The  mineral 
is  usually  mixed  with  other  ores  in  compact  masses,  but  it  also  forms 
handsome  crystals. 

The  composition  of  pyrargyrite  is  represented  by  the  formula  Ag3SbS3 
which  demands  17.82  per  cent  S.;  22.21  per  cent  Sb.;  59.97  per  cent  Ag. 
Many  specimens  contain  also  a  small  quantity  of  arsenic,  through  the 
admixture  of  the  isomorphous  compound  proustite.  The  analyses  given 
below  show  the  effect  of  the  intermixture  of  the  two  molecules. 


- 

S 

Sb 

As 

Ag 

Total 

Andreasberg,  Harz .  .  . 

•   1765 

22.36 

•   •   •   ■ 

59- 73 

99-77 

Zacatecas,  Mexico 

•   1774 

22.39 

■27 

60.04 

100.44 

Freiberg,  Saxony 

•   17-95 

18.58 

2.62 

60.63 

99.78 

The  crystals  of  pyrargyrite  are  rhombohedral  and  hemimorphic 
(ditrigonal  pyramidal  class),  with  an  axial  ratio  1  :  .8038.  They  are 
usually  quite  complex  and  are  often  twinned.  The  species  is  very  rich 
in  forms,  not  less  than  150  having  been  reported.  The  most  prominent 
of  these  are  ooP2(ii2o),  ooP(ioTo),  R(ioTi),  — ^R(oil2)  and  the 
scalenohedrons  R3(2i3i)  and  JR3(2I34)  (Fig.  49).  In  the  commonest 
twinning  law  the  twinning  plane  is  00  P2 (11 20)  and  the  composition 


118 


DESCRIPTIVE  MINERALOGY 


Fig.  49. — Crystal  of 
Pyrargyritc  with 
00  P2,  1 120  (a) 
and  —  JR,oil2(«). 


face  oP(ooi).  The  c  axes  in  the  twinned  portions  are  parallel  and  the 
00  P2  (11 20)  planes  coincident,  so  that  the  twin  at  a  hasty  glance  looks 
like  a  simple  crystal.    The  angle  10T1 A  ^101  =  71°  22'. 

The  cleavage  of  pyrargyrite  is  distinct  parallel  to  R(ioTi).    Its  frac- 
ture is  conchoidal  or  uneven.    The  mineral  is  apparently  opaque  and  its 

color  is  grayish  black  in  reflected  light,  but  is  trans- 
parent or  translucent  and  deep  red  in  transmitted 
light.  Its  streak  is  purplish  red.  For  lithium 
light  o>=  3.084,  €=2.881.  It  is  not  an  electrical 
conductor. 

In  the  closed  tube  the  mineral  fuses  easily  and 
gives  a    reddish    sublimate.     When   heated    with 
sodium  carbonate  on  charcoal  it  is  reduced  to  a 
globule  of   silver,  which,  when   dissolved  in  nitric 
acid,    yields    a    silver    chloride   precipitate   when 
treated  with  a  soluble  chloride.    The  mineral  dis- 
solves in  nitric  acid  with  the  separation  of  sulphur 
and  a  white  precipitate  of  antimony  oxide.     It  is   also  soluble  in   a 
strong  solution  of  KOH.     From  this  solution  HC1  precipitates  orange 
Sb2S3  (compare  proustite). 

The  color  and  streak  of  pyrargyrite,  together  with  its  translucency, 
distinguish  it  from  nearly  all  other  minerals.  Its  reaction  for  silver 
serves  to  distinguish  it  from  cuprite,  cinna.ar  and  realgar,  which  it  some- 
times resembles.  The  distinction  between  this  mineral  and  its  iso- 
morph,  proustite,  is  based  on  the  streak  and  the  reaction  for  anti- 
mony. 

Pyrargyrite  occurs  as  a  pseudomorph  after  native  silver.  On  the 
other  hand  it  is  occasionally  altered  to  pyrite  or  argentite,  and  some- 
times to  silver. 

Syntheses. — Microscopic  crystals  have  been  made  by  heating  in  a 
porcelain  tube,  metallic  silver  and  antimony  chlorides  in  a  current  of 
H2S,  and  by  the  action  of  the  same  gas  at  a  red  heat  on  a  mixture  of 
metallic  silver  and  melted  antimony  oxide. 

Occurrence,  Localities  and  Origin. — Pyrargyrite  occurs  in  veins  asso- 
ciated with  other  compounds  of  silver  and  sometimes  with  galena  and 
arsenic.  It  is  most  common  in  the  zone  of  secondary  enrichment  of 
silver  veins.  The  crystallized  variety  is  found  at  Andreasberg  in  the 
Harz;  at  Freiberg,  in  Saxony;  at  Pribram,  in  Bohemia;  at  many  places 
in  Hungary,  and  at  Chafiarcillo,  in  Chile.  The  massive  variety  is  worked 
as  an  ore  of  silver  at  Guanajuato  in  Mexico  and  in  several  of  the  western 
states,  as,  for  instance,  in  the  Ruby  district,  Gunnison  Co..  and  in  other 


SULPHO-SALTS  AND  SULPHO-FERRITES  119 

mining  districts  in  Colorado,  near  Washoe  and  Austin,  Nevada,  and  at 
several  points  in  Idaho,  New  Mexico,  Utah  and  Arizona. 

Uses. — The  mineral  is  an  important  ore  of  silver  in  Mexico  and  in 
the  western  United  States.  It  is  usually  associated  with  other  sulphur- 
bearing  ores  of  silver,  the  metal  being  extracted  from  the  mixture  by 
the  processes  referred  to  under  argentite. 

Proustite  (Ag3AsS3) 

Proustite,  or  light  ruby  silver,  is  isomorphous  with  pyrargyrite.  It 
differs  from  the  latter  mineral  in  containing  arsenic  in  place  of  antimony. 
It  occurs  both  massive  and  in  crystals,  and  like  pyrargyrite  is  an  ore  of 
silver. 

The  formula  above  given  demands  1943  per  cent  S;  15.17  per  cent 
As;  and  65.40  per  cent  silver..  The  analysis  of  a  specimen  from  Mexico 
yields  figures  that  correspond  very  nearly  to  these.  Crystals  from 
Chafiarcillo  contain  a  slight  admixture  of  the  antimony  compound. 

S 

Mexico 19 .  52 

Chafiarcillo,  Chile 19-64 

Like  pyrargyrite,  proustite  is  rhombohedral.  Its  crystals  are  pris- 
matic or  acute  rhombohedral.  The  forms  present  on  them  are  much 
less  numerous  than  those  on  the  corresponding 
antimony  compound,  the  predominant  ones  being 
ooP2(ii2o),  JR(ioT4),  -JR(oii2),  R3(2i3i), 
—|R4(3587)  and  other  scalenohedrons  (see  Fig. 
50).  Twins  are  common,  the  twinning  planes 
being  (1),  parallel  to  }R(iol4)  and  (2)  parallel  to 

R(ioTi).     The  angle  10T1  aTici  =  72°  12'.  FlG-  5o.-Crystal  of 

mi       1  £      -l  j  i_     j  £  Proustite    with    00  P2. 

The  cleavage,  fracture  and  haidness  of  prous-        -    ,  .        #T>1         ' 

£  t,    i_    j      II2°  W»   ~!R »  35S7 

tite  are  the  same  as  for  pyrargyrite.    Its  hard-     ^M)  and  -JR,oii2  (e). 

ness  is  2  and  its  density  is  about  5.6.    The  mineral 

is  transparent  or  translucent.     Its  color  is  grayish  black  by  reflected 

light  arid  scarlet  in  transparent  pieces  by  transmitted  light.    Under  the 

long-continued  influence  of  daylight  the  color  deepens  until  it  becomes 

darker  than  that  of  pyrargyrite.     Its  streak  is  cinnabar-red  to  brownish 

black.     Its  luster  is  adamantine.     It  is  a  nonconductor  of  electricity. 

For  sodium  light  01=3.0877,  €=  2.7924. 

In  the  closed  tube  proustite  fuses  easily  and  gives  a  slight  sublimate 


As 

Sb 

Ag 

Total 

14.98 

•      •      •      ■ 

65-39 

99.89 

I3-85 

I. 41 

65.06 

99.96 

120  DESCRIPTIVE  MINERALOGY 

of  white  arsenic  oxide.  In  its  other  chemical  properties  it  resembles 
pyrargyrite  except  that  it  gives  reactions  for  arsenic  where  this  mineral 
reacts  for  antimony,  and  yields  only  sulphur  when  dissolved  in  HNO3. 
From  its  solution  in  KOH  a  yellow  precipitate  of  AS2S3  is  thrown  down 
upon  the  addition  of  HC1  (compare  pyrargyrite). 

Proustite  differs  from  pyra.gyite  in  its  color,  transparency  and 
streak,  as  well  as  in  its  arsenic  reactions.  It  is  distinguished  from 
cinnabar  and  cuprite  (CuO)  by  the  arsenic  test. 

Syntheses. — Crystals  of  proustite  have  been  produced  by  reactions 
analogous  to  those  that  yield  pyrargyrite,  when  arsenic  compounds  are 
employed  in  place  of  antimony  compounds. 

Occurrence. — The  mineral  occurs  under  the  same  conditions  and  with 
the  same  associates  as  pyrargyrite  and  it  yields  the  same  alteration 
products  as  pyrargyrite. 

Localities  and  Uses. — Handsome  crystals  of  proustite  occur  at 
Freiberg  and  other  places  in  Saxony,  at  Wolfach  in  Baden,  at  Markirchen 
in  Alsace  and  at  Chaftarcillo  in  Chile.  It  is  associated  with  pyrargyrite 
and  with  other  ores  of  silver. 

In  the  western  United  States  it  is  quite  abundant,  more  particularly 
in  the  Ruby  district,  Colorado,  at  Poorman  lode  in  Idaho,  and  in  all  other 
localities  where  pyrargyrite  occurs.  In  many  it  is  mined  as  an  ore  of 
silver. 

Bournonite  ((Pb-Cu2)3(SbS3)2) 

Bournonite  is  a  comparatively  rare  mineral.  It  occurs  either  in 
compact  or  granular  masses  or  in  well  developed  crystals  of  a  steel- 
gray  color.  It  is  not  of  any  economic  importance  except  as  it  may  be 
mixed  with  other  copper  compounds  exploited  for  copper. 

Analyses  of  bournonite  from  two  localities  are  given  below: 

S  Sb 

I.  I9-36         23.57 
II.  19.78        23.80 

I.  Liskeard,  Cornwall,  England. 

II.  Felsdbanya,  Hungary. 

These  analyses  are  by  no  means  accurate,  but  they  show  the  compo- 
sition of  the  mineral  to  be  approximately  Pb,  Cu,  Sb  and  S,  in  which  the 
elements  are  combined  in  the  following  proportions:  S=ig.8  per  cent; 
Sb=  247  per  cent;  Pb  42.5  per  cent;  Cu  13  per  cent. 

Bournonite  crystals  are  orthorhombic  (rhombic  bipyramidal  class), 


As 

Pb 

Cu 

Fe 

Total 

•47 

41-95 

1327 

.68 

99  30 

•      •      • 

42.07 

12.82 

.20 

98.67 

SULPHO-SALTS  AND  SULPHO-FERRITES 


121 


with  alb:  £=.9380  :  1  :  .8969.  They  are  usually  tabular  (Fig.  51),  or 
short,  prismatic  in  habit,  and  are  often  in  repeated  twins  (Fig.  52),  with 
wheel-shaped  or  cross-like  forms.  The  principal  planes  observed  on 
them  are  oP(ooi),  Poo  (ioi),Poo  (on),  £P(ii2),  ooP(no),  00  Poo  (100) 
and  00  P  06  (010),  though  00  or  more  planes  are  known.  The  most  com- 
mon twinning  plane  is  00  P(no).    Angle  no  A  iTo=86°  20'. 

The  luster  of  the  mineral  is  brilliant  metallic.  Its  color  and  streak 
are  steel-gray.  Its  cleavage  is  imperfect,  parallel  to  00  P  06  (010)  and  its 
fracture  conchoidal  or  uneven.  Its  hardness  is  2.5-3  and  density  5.8. 
Like  most  other  metallic  minerals  it  is  opaque.  It  is  a  very  poor  con- 
ductor of  electricity. 

In  the  closed  tube  bournonite  decrepitates  and  yields  a  dark  red  sub- 
limate. In  the  open  tube,  and  on  charcoal,  it  gives  reactions  for  Sb,  S, 
Pb  and  Cu.    When  treated  with  nitric  acid  it  decomposes,  producing  a 


V 

**l 

■^ 

< 

*~^ 

a 

b 

m 

& 

m 

Fig.  51.  Fig.  52. 

Fig.  51. — Bournonite  Crystal  with  oP,  001  (r);  Poo  ,  101  (0);   JP,  112  (u)  and  Poo  , 

on  (»). 
Fig.  52. — Bournonite  Fourling  Twinned  about  00  p,  no  (m).    Form  c  same  as  in 

Fig.  51.     6=  00  P  00  (010)  and  a—  00  P  65  (100). 

blue  solution  of  copper  nitrate  that  turns  to  an  intense  azure  blue  when 
an  excess  of  ammonia  is  added.  In  this  solution  is  a  residue  of  sulphur 
and  a  white  precipitate  that  contains  lead  and  antimony. 

Bournonite  is  distinguished  from  most  other  minerals  by  its  reactions 
for  both  antimony  and  sulphur.  From  other  sulphantimonites  it  is 
distinguished  by  its  color,  hardness  and  density. 

On  long  exposure  to  the  atmosphere  bournonite  alters  to  the  car- 
bonates of  lead  (cerussite)  and  copper  (malachite  and  azurite). 

Synthesis, — Crystals  of  bournonite  have  been  obtained  by  the  action 
of  gaseous  H2S  on  the  chlorides  and  oxides  of  Pb,  Cu  and  Sb,  at  moderate 
temperatures. 

Occurrence. — The  mineral  occurs  principally  in  veins  with  galena, 
sphalerite,  stibnite,  chalcopyrite  and  tetrahedrite. 

Localities. — Good  crystals  are  found  in  the  mines  at  Neudorf,  Harz; 
at  Pribram,  in  Bohemia;  at  Felsobanya,  Kapnik  and  other  places 
in  Hungary,  and  at  various  places  in  Chile.     In  North  America  it  has 


122  DESCRIPTIVE  MINERALOGY 

been  found  at  the  Boggs  Mine  in  Yavapai  Co.,  Ariz.,  in  Montgomery 
Co.,  Ark.,  and  at  Marmora,  Hastings  Co.,  and  Darling,  Lanark  Co., 
Ontario. 

THE  SULPHDIARSENITES  AND  SULPHDIANTIMONITES 

A  large  number  of  sulpho-salts  we  apparently  salts  of  acids  that 
contain  two  or  more  atoms  of  As  or  Sb  in  the  molecule.  These  acids 
may  be  regarded  as  derived  from  the  ortho  acids  by  the  abstraction  of 
H2S,  thus:  The  arsenious  acid  containing  two  atoms  of  As  may  be 
thought  of  as  2H3ASS3— H2S  =  H4As2S6.  Acids  with  larger  proportions 
of  arsenic  may  be  regarded  as  derived  in  a  similar  manner  from  three  or 
more  molecules  of  the  ortho  acid.  Only  a  few  of  these  salts  are  common 
as  minerals.  Among  the  more  common  are  two  that  are  lead  salts  of 
derivatives  of  sulpharsenious  and  sulphantimonous  acids. 

Jamesonite  (Pb2Sb2S5)  and  Dufrenoysite  (Pb2As2S5) 

Jamesonite  and  dufrenoysite  are  lead  salts  of  the  acids  H4SD2S5  and 
H4AS2S5.  Both  minerals  occur  in  acicular  and  columnar  orthorhombic 
crystals  and  in  fibrous  and  compact  masses  of  lead-gray  color.  Their 
cleavage  is  parallel  to  the  base.  The  minerals  are  brittle  and  have  an 
uneven  to  conchoidal  fracture.  Their  hardness  is  2-3  and  density 
5.5-6.  The  streak  of  jamesonite  is  grayish  black,  and  of  dufreynosite 
reddish  brown.  Both  minerals  are  easily  fusible.  They  are  soluble  in 
HO  with  the  evolution  of  H2S,  giving  a  solution  from  which  acicular 
crystals  of  PbCk  separate  on  cooling.  They  are  decomposed  by  HNO3, 
with  the  separation  of  a  white  basic  lead  salt.  They  are  found  in  veins 
with  antimony  and  sulphide  ores  abroad  and  at  several  points  in  Nevada, 
and  in  the  antimony  mines  in  Sevier  Co.,  Arkansas. 

THE  SULPHARSENATES  AND   SULPHANTIMONATES 

The  sulpharsenates  are  salts  of  sulpharsenic  acid,  H3ASS4,  and  the 
sulphantimonates,  the  salts  of  the  corresponding  antimony  acid,  HaSbS4. 
These  compounds  are  much  less  numerous  among  the  minerals  than  the 
sulpharsenites  and  sulphantimonites.  Moreover,  no  member  of  the 
former  groups  is  as  common  as  several  of  the  members  of  the  latter. 
The  most  important  member  is  the  mineral  enargite  (C113ASS4)  an  ortho- 
sulpharsenate,  which  in  a  few  places  is  wrought  as  a  copper  ore. 


SULPHO-SALTS  AND  SULPHO-FERRITES 


123 


Enargite  (Cu3AsS4) 

Enargite,  though  a  rare  mineral,  is  so  abundant  at  a  few  points  that 
it  has  been  mined  as  an  ore  of  copper. 

Theoretically,  the  mineral  is  S  =  32.6,  As=i9.i,  01=4.83.  Most 
specimens,  however,  contain  an  admixture  of  the  isomorphous  anti- 
mony compound,  famatinite,  and  consequently  show  the  presence  of 
antimony.    A  specimen  from  the  Rams  Mine,  Butte,  Montana,  yielded 


s 

As 

Sb 

Cu 

Fe 

Zn 

Ins 

Total 

3J-44 

17.91 

1.76 

48.67 

•33 

.10 

.11 

100.32 

Fig.  53. — Enargite  Crys- 
tal with  00  P,  no  (m); 
oopoo  ,  100(a);  00  P^, 
1 20  (A)  and  oP,  001  (c). 


The  mineral  crystallizes  in  the  orthorhombic  system  (bipyramidal 
class),  in  crystals  with  an  axial  ratio  .8694  :  1  :  .8308.  Their  habit  is 
usually  prismatic,  and  they  are  strongly  striated 
vertically.  The  crystals  are  usually  highly  modi- 
fied, with  the  following  forms  predominating: 
00  P56  (100),  ooP(no),  ooP2(i2o),  ooP3(i3o), 
00  P  06  (010),  and  oP(ooi)  (Fig.  53).  Stellar  trill- 
ings, with  ooP2(i2o)  the  twinning  plane,  have  a 
pseudohexagonal  habit.  The  mineral  occurs  also 
in  columnar  and  platy  masses. 

Enargite  possesses  a  perfect  prismatic  cleavage 
and  an  uneven  fracture.  It  is  opaque  with  a 
grayish  black  color  and  streak.  Its  hardness  is  3 
and  density  4.4.    It  is  a  poor  electrical  conductor. 

It  is  easily  fusible  before  the  blowpipe.  When  roasted  on  charcoal 
it  gives  the  reactions  for  S  and  As,  and  thq  roasted  residue  when 
moistened  with  HC1  imparts  to  the  flame  the  azure-blue  color  char- 
acteristic of  copper.  In  the  closed  tube  it  decrepitates  and  gives  a 
sublimate  of  S.  When  heated  to  fusion  it  yields  a  sublimate  of  arsenic 
sulphide.    The  mineral  is  soluble  in  aqua  regia. 

Enargite  is  easily  recognized  by  its  crystallization  and  blowpipe 
reactions. 

Occuf fence. Tr-Eneirgite  is  associated  with  other  copper  ores  in  veins 
filled  by  magmatic  water  at  intermediate  depths  and  in  a  few  replace- 
ment deposits. 

Localities. — Although  not  widely  distributed,  enargite  occurs  in  large 
quantities  in  the  copper  mines  near  Morococha,  Peru;  Copiapo,  Chile; 
in  the  province  of  La  Rioja,  Argentine;  on  Luzon,  Philippine  Islands, 


124  DESCRIPTIVE  MINERALOGY 

and  in  the  United  States,  at  Butte,  Montana;   in  the  San  Juan  Moun- 
tains, Colorado,  and  in  the  Tintic  District,  Utah. 

Uses. — It  is  smelted  as  an  ore  of  copper.  At  the  Butte  smelter  it 
furnishes  the  arsenic  that  is  separated  from  the  smelter  fumes  and  placed 
upon  the  market  as  arsenic  oxide  (see  p.  113). 

THE  BASIC  SULPHO-SALTS 

The  basic  sulpho-salts  are  compounds  in  which  there  is  a  greater 
percentage  of  the  basic  elements  (metals,  etc.),  present  than  is 
necessary  to  replace  all  the  hydrogen  of  the  ortho  acids.  Thus,  the 
copper  orthosulpharsenate,  enargite,  is  CU3ASS4  The  mineral  steph- 
anite  is  AgsSbS4  and  the  pure  silver  polybasite  AggSbSo. 

Since  three  atoms  of  Ag  are  sufficient  to  replace  all  the  hydrogen 
atoms  in  the  normal  acid  containing  one  atom  of  antimony  and  the 
quantities  of  silver  present  in  stephanite  and  polybasite  are  in  excess 
of  this  requirement,  the  two  minerals  are  described  as  basic.  The 
exact  relations  of  the  atoms  to  one  another  in  the  molecules  are 
not  known. 

Although  the  number  of  basic  sulpho-salts  occurring  as  minerals  is 
large  only  four  arc  common.    These  are: 

StepJtanite  AgoSbS4  Orthorhombic 

Polybasite  (Ag-Cu)9SbSo  Monoclinic 

Tetrahedrite  (R")4Sb2S7  Isometric 

Tennantite  (R")4As2S7  Isometric 

Stephanite  (Ag5SbS4) 

Stephanite,  though  a  comparatively  rare  mineral,  is  an  important  ore 
of  silver  in  some  camps.  It  occurs  massive,  in  disseminated  grains  and 
as  aggregates  of  small  crystals.  Analyses  indicate  a  composition  very 
close  to  the  requirements  of  the  formula  AgsSbS4. 

S  Sb         Ag       AsandCu     Total 

Theoretical 16.28     15.22    68.50         ....  100.00 

Crystals,  Chaiiarcillo,  Chile  16.02     15.22     68 .  65  tr  99 .  89 

Stephanite  crystallizes  in  hemimorphic  orthorhombic  crystals  (rhom- 
bic pyramidal  class),  with  an  axial  ratio  .6291  :  1  :  .6851.  The  crystals 
are  highly  modified,  125  forms  having  been  identified  upon  them.  They 
have  usually  the  habit  of  hexagonal  prisms,  their  predominant  planes 


SULPHO-SALTS  AND  SULPHO-FERRITES  125 

being  ooP(no)  and  ooP 06(010),  terminated  by  oP(ooi),  P(in)  and 
2  P  06  (021)  at  one  or  the  other  end  of  the  c  axis  (Fig.  54).  Twins  are 
common,  with  00  P(no)  and  oP(ooi)  the  twinning  planes. 

The  mineral  is  black  and  opaque  and  its  streak  is  black.     Its  hard- 
ness is  2  and  density =6. 2  — 6.3.     It  cleaves 
parallel  to  00  P  06  (010)  has  an  uneven  frac- 
ture, and  is  a  poor  conductor  of  electricity. 

On  charcoal  stephanite  fuses  very  easily 
to  a  dark  gray  globule,  at   the  same  time 
yielding  the  white  fumes  of  antimony  oxide   Fig.  54.— Stephanite  Crystal 
and  the  pungent  odor  of  SO2.    Under  the      with  oP,  001  (c);   00  p«, 

reducing   flame    the  globule   is   reduced   to      OI°  )**  °°-  S1IC    ,i\     ' 

..  332  (P),  2P00 ,  021  (d). 

metallic    silver.     The    mineral    dissolves  in 

dilute  nitric  acid  and  this  solution  gives  a  white  precipitate  with  HC1. 

Stephanite  is  easily  distinguished  from  other  black  minerals  by  its 
easy  fusibility,  its  crystallization,  and  its  reactions  for  Ag,  Sb  and  S. 

Localities. — The  mineral  is  associated  with  ether  silver  ores  in  the 
zone  of  secondary  enrichment  of  veins  at  Freiberg,  Saxony;  Joachimsthal 
and  Pribram,  Bohemia;  the  Comstock  Lode  and  other  mines  in  the 
Rocky  Mountain  region  and  at  many  points  in  Mexico  and  Peru. 

Uses. — It  is  mined  together  with  other  compounds  as  an  ore  of  silver^ 
It  is  particularly  abundant  in  the  ores  of  the  Comstock  Lode,  Nev.,  and 
of  the  Las  Chispas  Mine,  Sonora,  Mex. 

Polybasite  ((Ag-Cu)9SbS6) 

Polybasite  is  the  name  usually  applied  to  the  mixture  of  basic  sulph- 
antimonites  and  suipharsenites  of  the  general  formula  R'9(Sb.As)Se,  in 
which  R'  =  Ag  and  Cu.  More  properly  the  name  is  applied  to  the  anti- 
monite,  and  the  corresponding  arsenite  is  designated  as  pearceiie.  Sev- 
eral typical  analyses  follow: 

Fe       Pb       Ins       Total 

....        ...        ...  99 ' "9 

I.05        ...         .42  99.85 

....  ...  ...  1UO .  V/U 

76       ...        100.18 

I.  Pearceite,  Veta  Rica  Mine,  Sierra  Mojada,  Mexico. 
II.  Crystals  of  pearceite,  Drumlummon  Mine,  Marysville,  Montana. 

III.  Polybasite,  Santa  Lucia  Mine,  Guanajuato,  Mexico. 

IV.  Polybasite,  Quespisiza,  Chile. 


s 

As 

Sb         Ag 

Cu 

I. 

17.46 

7-56 

59-22 

1565 

II. 

17.71 

7-39 

55.17 

18. n 

III. 

15.43 

■50 

10.64    68.39 

5i3 

IV. 

16.37 

388 

515     67.95 

6.07 

126  DESCRIPTIVE  MINERALOGY 

The  crystallization  of  the  two  minerals,  which  are  completely  isomor- 
phous,  is  monoclinic  (prismatic  class).    Their  axial  ratios  are: 

Pearceite,       a  :  b  :  c=  1.7309  :  1  :  1.6199       £=90°  9' 
Polybasite,  =17309  :  1  :  1.5796        £=90° 

The  crystals  are  commonly  tabular  or  prismatic,  with  a  distinct 
hexagonal  habit.  The  prominent  forms  are  oP(ooi),  P(in)  and 
2P  06  (20T).  Contact  twinning  is  common,  with  00  P(no)  the  twinning 
plane,  and  oP(ooT)  the  composition  plane. 

Both  minerals  are  nearly  opaque.  Except  in  very  thin  splinters 
they  are  steel-gray  to  iron-black  in  color.  Very  thin  plates  are  trans- 
lucent and  cherry-red.  Their  streaks  are  black.  Their  cleavage  is 
perfect  parallel  to  oP(ooi)  and  their  fracture  uneven.  Their  hardness 
is  2-3,  and  density  6-6.2. 

Both  minerals  are  easily  fusible.  They  usually  exhibit  the  reactions 
for  Ag,  Sb,  As  and  S. 

They  are  readily  distinguished  from  all  other  minerals  but  silver 
sulpho-salts  by  their  blowpipe  reactions.  From  these  they  are  distin- 
guished by  their  crystallization.  Pearceite  and  polybasite  are  distin- 
guished from  one  another  by  the  relative  quantities  of  As  and  Sb  they 
contain. 

Occurrence. — Both  minerals  occur  in  the  zone  of  secondary  enrich- 
ment in  veins  of  silver  sulphides. 

Localities. — Polybasite  was  an  important  ore  of  silver  in  the  Comstock 
Lode,  Nevada.  It  is  at  present  mined  with  other  silver  ores  at  Ouray, 
Colorado,  at  Marysville,  Montana,  at  Guanajuato,  Mexico,  and  at 
various  points  in  Chile.  Good  crystals  occur  at  Freiberg*  Saxony,  at 
Joachimsthal,  Bohemia,  and  in  the  mines  in  Colorado,  Mexico  and  Chile. 

TETRAHEDRITE  GROUP 

The  name  tetrahedrite  is  given  to  a  mixture  of  basic  sulphanti- 
monites  and  sulpharsenites  crystallizing  together  in  isometric  forms  with 
a  distinct  tetrahedral  habit  (hextetrahedral  class).  The  isomorphism 
is  so  complete  that  all  gradations  between  the  various  members  of  the 
group  are  frequently  met  with.  The  arsenic-bearing  member  of  the 
series  is  known  as  tennantite  and  the  corresponding  antimony  member  as 
tetrahedrite.    The  latter  is  the  more  common. 

The  following  six  analyses  of  tetrahedrite  will  give  some  idea  of  the 
great  range  in  composition  observed  in  the  species. 


SULPHO-SALTS  AND  SULPHO-FERRITES  127 

S        Sb  As      Cu  Fe      Zn       Ag       Hg     Pb  Total 

I.  27.60  25.87  tr     35.85  2.665.15     2.30  99.43 

II.  23.51  17.21  7.67  42.00  8.28     .49       .55  99.71 

III.  24.44  27.60  ....   27.41  4.27  2.31  14.54  IOOS7 

IV.  24.8930.18    tr     32.80    5.85 07  5.57 99.36 

V.  21.67  24.72  ....  33.53       .56 1.80  16.23      98.51 

I.  Newburyport,  Mass. 
II.  Cajabamba,  Peru. 

III.  Star  City,  Nev. 

IV.  Pontes,  Hungary. 
V.  Arizona. 

Upon  examination  these  are  found  to  correspond  approximately  to 
the  formula  R"4Sb2S7,  in  which  the  R"  is  CU2,  Pb,  Fe,  Zn,  Hg,  Ag2  and 
sometimes  Co  and  Ni.  When  R  is  replaced  entirely  by  copper,  the 
formula  (CugSbaSz)  demands  23.1  per  cent  S,  24.8  per  cent  Sb  and  52.1 
per  cent  Cu. 

Analyses  of  tennantite  yield  analogous  results  that  may  be  repre- 
sented by  the  formula  CU8AS2S7  which  demands  26.6  per  cent  S,  20.76 
per  cent  As  and  52.64  per  cent  Cu. 

Analyses  of  even  the  best  crystallized  specimens  rarely  yield  As  or 
Sb  alone.  Moreover,  nearly  all  show  the  presence  of  Zn  in  notable 
quantity.  The  great  variation  noted  in  the  composition  of  different 
specimens  which  appear  to  be  pure  crystals  has  led  to  the  proposal  of 
other  formulas  than  those  given  above — some  being  simpler  and  others 
more  complex.  It  is  possible  that  the  variation  may  be  explained  as 
due,  in  part,  to  some  kind  of  solid  solution,  rather  than  as  the  result 
solely  of  isomorphous  replacement.  It  is  more  probable,  however,  that 
it  is  due  to  the  intergrowth  of  notable  quantities  of  various  sulphides 
with  the  sulpho-salts. 

There  is  still  considerable  confusion  in  the  proper  naming  of  the  mem- 
bers of  the  series,  but  generally  the  forms  composed  predominantly  of 
Cu,  Sb  and  S  with  or  without  Zn  are  known  as  tetrahedrite  and  those 
containing  As  in  place  of  Sb  as  tennantite,  although  several  authors 
confine  the  use  of  the  latter  term  to  arsenical  tetrahedrites  containing  a 
notable  quantity  of  iron. 

Since  the  members  of  the  tetrahedrite  series  often  contain  a  large 

* 

quantity  of  metals  other  than  Cu  and  Zn  the  group  has  been  so  sub- 
divided as  to  indicate  this  fact.  Thus,  there  are  argentiferous,  mercurial 
and  plumbiferous  varieties  of  tetrahedrite.  Some  of  these  varieties  are 
utilized  as  ores  of  the  metals  that  replace  the  copper  and  zinc  in  the  more 


Pb 

Co 

Total 

.11 

•  •  •  • 

100.15 

•  •  • 

•       •      •       B 

99.21 

•  •  • 

I.  20 

101.07 

128  DESCRIPTIVE  MINERALOGY 

common  varieties.  The  relations  of  the  ordinary  (II)  and  the  bis- 
muthif erous  tennantites  (III)  to  tetrahedrite  (I)  are  shown  by  the  fol- 
lowing three  analyses: 

S         As         Sb         Bi  Cu  Fe         Ag 

I.     24.48        tr  28.85     45-39  I-32 

II.     26.61  19.03  51.62  1.95 

III.     29.10  11.44  2.19     13.07  37.52  6.51        .04 

I.  Fresney  d'Oisans,  France. 
II.  Cornwall,  England. 
III.  Cremenz,  Switzerland. 

The  crystals  of  both  tetrahedrite  and  tennantite  are  tetrahedral  in 

habit,  the  principal  forms  on  them  consisting  of  the  simple  tetrahedron 

2O2  |0     _ 

and  complex  tetrahedrons  such  as (211), (332)  together  with 

2  2 

the  dodecahedron,  ooO(no)  and  the  cube,   00  O  00  (100).      (Fig.   55.) 

Twins  are  common  with  O(ni)  the  twinning 

plane.    These   are   sometimes   contact  twins 

and  sometimes  interpenetration  twins.     Some 

crystals  are  very  complicated,  because  of  the 

presence  on  them  of  a  great  number  of  forms. 

The  total  number  of  distinct  forms  that  have 

been   identified   is    about  90.     The  mineral 

_,        occurs  also    in  granular,   dense  and    earthy 
Fig.  <«>. — Tetrahedrite  Crys- 

0  masses, 

tal  with  -  in  (0);   00 o,        The  fracture  of  the  tetrahedrites  is  uneven, 
no  (d)  and  sO,  332  (»).      Their  hardness  varies  between  3  and  4.5  and 

their  density  between  4.4  and  5.1.  Their  color 
is  between  dark  gray  and  iron-black,  except  in  thin  splinters,  which 
sometimes  exhibit  a  cherry-red  translucency.  Their  streak  is  like  their 
color.    All  tetrahedrites  are  thermo-electric. 

The  chemical  properties  of  the  different  varieties  of  tetrahedrites 
vary  with  the  constituents  present.  All  give  tests  for  sulphur  and  for 
either  antimony  or  arsenic,  and  all  show  the  presence  of  copper  in  a 
borax  bead.  The  reactions  of  other  metals  that  may  be  present  may 
be  learned  by  consulting  pages  483-494. 

The  crystals  of  tetrahedrite  are  so  characteristic  that  there  is  little 
danger  of  confusing  the  crystallized  mineral  with  other  minerals  of  the 
same  color.  The  massive  forms  resemble  most  clearly  arsenopyrite, 
coba'tiU,  bournonite  and  chalcociie.     From  these  the  tetrahedrites  are 


SULPHO-SALTS  AND  SULPHO-FERRITES  129 

best  distinguished  by  their  hardness,  together  with  their  blowpipe  reac- 
tions. 

Tetrahedrite  appears  to  suffer  alteration  quite  readily,  since  pseudo- 
morpbs  of  several  carbonates  and  sulphides  after  tetrahedrite  crystals 
are  well  known. 

Syntheses. — Crystals  of  the  tetrahedrites  have  been  made  by  passing 
the  vapors  of  the  chlorides  of  the  metals  and  the  chlorides  of  arsenic  or 
antimony  and  H2S  through  red-hot  porcelain  tubes.  They  have  also 
been  observed  in  Roman  coins  that  had  lain  for  a  long  time  in  the  hot 
springs  of  Bourbonne-les-Bains,  Haute-Marne,  France. 

Occurrence. — The  tetrahedrites  are  very  common  in  the  zone  of 
secondary  enrichment  of  sulphide  veins  and  in  impregnations.  They 
occur  associated  with  chalcopyrite,  pyrite,  sphalerite,  galena  and  other 
silver,  lead  and  copper  ores  in  nearly  all  regions  where  the  sulphide  ores 
of  these  metals  are  found.  They  occur  also  as  primary  constituents  of 
veins  of  silver  ores,  where  they  were  deposited  by  magmatic  waters. 

Localities. — In  the  United  States  tetrahedrite  occurs  at  the  Kellogg 
Mines,  ten  miles  north  of  Little  Rock,  Arkansas;  near  Central  City  and 
at  Georgetown,  Colorado;  in  the  Ruby  and  other  mining  districts  in 
the  same  State;  at  the  De  Soto  Mine  in  Humboldt  Co.,  Nevada,  and 
at  several  places  in  Montana,  Utah  and  Arizona.  It  is  found  also  in 
British  Columbia  and  in  Mexico,  and  at  Broken  Hill,  New  South  Wales. 

The  arsenical  tetrahedrites  are  not  quite  as  common  as  is  the  anti- 
monial  variety.  Excellent  crystals  occur  in  the  Cornish  Mines,  at 
Freiberg  in  Saxony,  at  Skutterud  in  Norway,  and  at  Capelton, 
Quebec. 

Uses. — The  mineral  is  used  to  some  extent  as  an  ore  of  silver  or  of 
copper,  the  separation  of  the  metals  being  effected  in  the  same  way  as 
in  the  case  of  the  sulphides  of  these  substances. 

THE  SULPHO-FERRITES 

Only  two  sulpho-ferrites  are  sufficiently  important  to  merit  descrip- 
tion here.  Both  of  these  are  copper  compounds  and  both  are  used  as 
ores  of  this  metal,  one — chalcopyrite — being  one  of  the  most  important 
ores  of  the  metal  at  present  worked. 

The  first  of  these  minerals  discussed,  bornite,  is  a  basic  salt  of 
the  acid  HaFeSa;  the  second  is  the  salt  of  the  derived  acid  HFeS2, 
which  may  be  regarded  as  the  normal  acid  from  which  one  molecule  of 
H2S  has  been  abstracted  (see  p.  116). 


130  DESCRIPTIVE  MINERALOGY 

Bornite  (Cu5FeS4) 

Bornite,  known  also  as  horseflesh  ore  because  of  its  peculiar  purplish- 
red  color,  is  found  usually  massive.  In  Montana  and  in  Chile  it  con- 
stitutes an  important  ore  of  copper. 

Bornite  is  probably  a  basic  sulpho-ferrite,  though  analyses  yield 
results  that  vary  quite  widely,  especially  in  the  case  of  massive  varieties. 
This  variation  is  due  to  the  greater  or  less  admixture  of  copper  sulphides, 
mainly  chalcocite,  with  the  bornite.  The  theoretical  composition  of  the 
mineral  is  25.55  S,  63.27  Cu,  and  1 1.18  Fe.  The  analyses  of  a  crystallized 
variety  from  Bristol,  Conn.,  and  of  a  massive  variety  from  the  Bruce 
Mines,  Ontario,  follow: 

S  Cu         Fe       Ins     Total 

Bristol,  Conn 25.54    63.24    11.20     ...     99.98 

Bruce  Mines,  Ont 2S-39    62.78    11.28     .30    99.75 

The  crystallization  of  bornite  is  isometric  (hexoctahedral  class),  in 
combinations  of  00  O  00  (ico),  00  0(no),0(in),  and  sometimes  202(211). 
Crystals  often  form  interpenetration  twins,  with  O  the  twinning  plane. 

The  fracture  of  the  mineral  is  conchoidal,  its  hardness  3  and  density 
about  5.  On  fresh  fracture  the  color  varies  from  a  copper-red  to  a  pur- 
plish brown.  Upon  exposure  alteration  rapidly  takes  place  covering 
the  mineral  with  an  iridescent  purple  tarnish.  Its  streak  is  grayish 
black.     It  is  a  good  conductor  of  electricity. 

Chemically,  the  mineral  possesses  no  characteristics  other  than  those 
to  be  expected  from  a  compound  of  iron,  copper  and  sulphur.  It  dis- 
solves in  nitric  acid  with  the  separation  of  sulphur. 

It  is  easily  recognized  by  its  purplish  brown  color  on  fresh  fractures 
and  its  purple  tarnish. 

Bornite  alters  to  chalcopyrite,  chalcocite,  covellite,  cuprite  (CU2O), 
chrysocolla  (CuSi03  •  2H2O)  and  the  carbonates,  malachite  and  azurite. 
On  the  other  hand,  bornite  pseudomorphs  after  chalcopyrite  and  chal- 
cocite are  not  uncommon. 

Syntheses. — Roman  copper  coins  found  immersed  in  the  water  of 
warm  springs  in  France  have  been  partly  changed  to  bornite.  Crystals 
have  been  formed  by  the  action  of  H2S  at  a  comparatively  low  tempera- 
ture (ioo°-2oo°  C),  upon  a  mixture  of  CU2O,  CuO  and  Fe203 

Occurrence  and  Origin. — Bornite  is  usually  associated  with  other 
copper  ores  in  veins  and  lodes,  where  it  is  in  some  cases  a  primary  min- 
eral deposited  by  magmatic  waters  and  in  others  a  secondary  mineral 
produced  in  the  zone  of  enrichment  of  sulphide  veins.    It  also  sometimes 


SULPHO-SALTS  AND  SULPHO-FERRITES  131 

impregnates  sedimentary  rocks,  where  its  origin  is  part  due  to  contact 
action. 

Localities. — The  crystallized  mineral  occurs  near  Redruth,  Cornwall, 
Eng.,  and  at  Bristol,  Conn.  The  massive  mineral  is  found  at  many 
places  in  Norway  and  Sweden.  It  is  the  principal  ore  of  some  of  the 
Bolivian,  Chilian,  Peruvian  and  Mexican  mines  and  of  the  Canadian 
mines  near  Quebec.  In  the  United  States  it  has  been  mined  at 
Bristol,  Conn.,  and  at  Butte,  Montana, 

Uses. — Bornite  is  mined  with  chalcopyrite  and  other  copper  com- 
pounds as  an  ore  of  this  metal. 

Chalcopyrite  (CuFeS2) 

From  an  economic  point  of  view  this  mineral  is  the  most  important 
of  the  sulpho-salts,  as  it  is  one  of  the  most  important  ores  of  copper 


Fig.  56.  Fig.  5;. 

Fig.  56.— Chalcopyrite  Crystal  with  P,  111  (J>);  -P, 
Fig.  57.— Chalcopyrite  Crystal  with  — ,  772  (*)  and 

sometimes  approaches  w  I'(iio)  and  x  approach! 

Fig.  58.— Chalcopyrite  Twinned  about  P(m}. 

known.  It  occurs  both  massive  and  crystallized.  From  its  similarity 
to  pynte  in  appearance  it  is  often  known  as  copper  pyrites. 

Crystallized  specimens  of  chalcopyrite  contain  35  per  cent  S,  34.5 
per  cent  Cu  and  30.5  per  cent  Fe,  corresponding  to  the  formula  CuFeSa, 
i.e.,  a  copper  salt  of  the  acid  HFeSj.  The  mineral  often  contains  small 
quantities  of  intermixed  pyrite.  It  also  contains  in  some  instances 
selenium,  thallium,  gold  and  silver. 

The  crystallization  of  chalcopyrite  is  in  the  sphenoidal,  hemihedral 
division  of  the  tetragonal   system   (tetragonal   scalenohedron   class). 


132  DESCRIPTIVE  MINERALOGY 

p 

The  crystals  are  usually  sphenoidal  in  habit  with  the  sphenoids  -(in), 

2 
|P 

and  — (332)  the  predominant  forms  (Figs.  56  and  57).    In  addition  to 
2 

these  there  are  often  present  also  00  P  00  (100),  00  P(no),  2P  00  (201), 

xp 
and  a  very  acute  sphenoid  that  is  approximately  —(772),  supposed  to  be 

2 

p 

due  to  the  oscillation  of  00  P(no)  and  —(in)  (Fig.  57).    Twins  are  quite 

common,  with  the  twinning  plane  parallel  to  P  (Fig.  58).  The  plus 
faces  of  the  sphenoid  are  often  rough  and  striated,  while  the  minus  faces 
are  smooth  and  even. 

The  fracture  of  the  mineral  is  uneven.  Its  hardness  is  3.5-4  and 
density  about  4.2.  Its  luster  is  metallic  and  color  brass-yellow.  Old 
fracture  surfaces  are  often  tarnished  with  an  iridescent  coating.  Its 
streak  is  greenish  black.    It  is  an  excellent  conductor  of  electricity. 

On  charcoal  the  mineral  melts  to  a  magnetic  globule.  When  mixed 
with  Na2C03  and  fused  on  charcoal,  a  copper  globule  containing  iron 
results.  When  treated  with  nitric  acid  it  dissolves,  forming  a  green 
solution  in  which  float  spongy  masses  of  sulphur.  The  addition  of 
ammonia  to  the  solution  changes  it  to  a  deep  blue  color  and  at  the  same 
time  causes  a  precipitate  of  red  ferric  hydroxide. 

From  the  few  brassy  colored  minerals  that  resemble  it,  chalcopyrite 
is  distinguished  by  its  hardness  and  streak. 

W"hen  subjected  to  the  action  of  the  atmosphere  or  to  percolating 
atmospheric  water  chalcopyrite  loses  its  iron  component  and  changes 
to  covellite  and  chalcocite.  The  iron  passes  into  limonite.  Bornite, 
copper  and  pyrite  are  also  frequent  products  of  its  alteration.  In  the 
oxidation  zone  of  veins  it  yields  limonite,  the  carbonates,  malachite  and 
azurite,  and  cuprite  (CU2O).  When  exposed  to  the  leaching  action  of 
water,  limonite  alone  may  remain  to  mark  the  outcrop  of  veins,  the 
copper  being  carried  downward  in  solution  to  enrich  the  lower  portions 
of  the  vein.  The  deposit  of  limonite  on  the  surface  is  known  as 
"  gossan." 

Syntheses. — Crystals  of  chalcopyrite  have  been  produced  by  the 
action  of  H2S  upon  a  moderately  heated  mixture  of  CuO  and  Fe203 
enclosed  in  a  glass  tube.  The  mineral  has  also  been  made  by  the  action 
of  warm  spring  waters  upon  ancient  copper  coins.  It  is  also  a  fairly 
common  product  of  roasting-oven  operations. ' 

Occurrence  and  Origin. — Chalcopyrite  is  widely  disseminated  as  a 
primary  vein  mineral,  and  is  often  found  in  nests  in  crystalline  rocks. 


SULPHO-SALTS  AND  SULPHO-FERRITES  133 

It  also  impregnates  slates  and  other  sedimentary  rocks,  schists  and 
altered  igneous  rocks  where,  in  some  cases,  it  is  a  contact  deposit 
and  in  others  is  original.  It  is  also  formed  by  secondary  processes  caus- 
ing enrichment  of  copper  sulphide  veins.  Its  most  common  associ- 
ates are  galena,  sphalerite  and  pyrite»  It  is  the  principal  copper  ore 
in  the  Cornwall  mines,  where  it  is  associated  with  cassiterite  (SnCfe), 
galena  and  other  sulphides.  It  is  also  the  important  copper  ore  of 
the  deposits  of  Falun,  Sweden,  of  Namaqualand  in  South  Africa, 
those  near  Copiap6  in  Chile,  those  of  Mansfeld,  Germany,  of  the  Rio 
Tinto  district  in  Spain,  of  Butte  and  other  places  in  Montana,  and  of 
the  great  copper-producing  districts  in  Arizona,  Utah  and  Nevada. 

Crystals  occur  near  Rossie,  Wurtzboro  and  Edenville,  N.  Y.,  at  the 
French  Creek  Mines,  Chester  Co.,  Perm.,  near  Finksburg,  Md.,  and  at 
many  other  places. 

Extraction. — The  mineral  is  concentrated  by  mechanical  methods. 
The  concentrates  are  roasted  at  a  moderately  high  temperature,  the  iron 
being  transformed  into  oxides  and  the  copper  partly  into  oxide  and 
partly  into  sulphide.  Upon  further  heating  with  a  flux  the  iron  oxide 
unites  with  this  to  form  a  slag  and  the  copper  sulphide  melts,  and  collects 
at  the  bottom  of  the  furnace  as  "  matte,"  which  consists  of  mixed  copper 
and  copper  sulphide.  This  is  roasted  in  a  current  of  air  to  free  it  from 
sulphur.  By  this  process  all  of  the  copper  is  transformed  into  the  oxide, 
which  may  be  converted  into  the  metal  by  reduction.  The  metal  is 
finally  refined  by  electrical  processes.  Much  of  the  copper  obtained 
from  chalcopyrite  contains  silver  or  gold,  or  both,  which  may  be  recov- 
ered by  any  one  of  several  processes. 

Uses. — A  large  portion  of  the  ccpper  produced  in  the  world  is  obtained 
by  the  smelting  of  chalcopyrite  and  the  ores  associated  with  it. 

Production. — The  world's  total  product  of  copper  has  been  referred 
to  in  another  place  (p.  55).  Of  this  total  (2,251,300,000  lb.)  the  United 
States  supplied,  in  191 2, 1,243,300,000  lb.,  of  which  about  1,000,000,000 
lb.  were  obtained  from  sulphide  ores.  Arizona  and  Montana  produced 
the  greater  portion  of  this  large  quantity,  the  former  contributing  about 
359,000,000  lb.  to  the  aggregate,  and  the  latter  308,800,000  lb.  Out- 
side of  the  United  States  the  most  important  copper-producing  countries 
are  Mexico,  Japan,  Spain  and  Portugal,  Australia,  Chile,  Canada, 
Russia,  Peru  and  Germany,  in  the  order  named.  Practically  all  of  this 
copper,  except  that  from  Japan  and  Mexico,  is  extracted  from  sulphide 
ores. 


CHAPTER  VI 

THE   CHLORIDES.   BROMIDES,   IODIDES  AND   FLUORIDES 

The  salts  belonging  to  this  group  are  compounds  of  metals  with 
hydrochloric  (HC1),  hydrobromic  (HBr),  hydriodic  (HI)  and  hydro- 
fluoric (HF)  acids.  Only  a  few  are  of  importance.  Of  these  some  are 
simple  chlorides,  others  are  simple  fluorides,  others  are  double  chlorides 
or  fluorides  (i.e.  cryolite,  AlF3+3NaF),  and  others  are  double  hydrox- 
ides and  chlorides  (atacamite). 

THE  CHLORIDES 

The  simple  chlorides  crystallize  in  the  isometric  system,  but  in  differ- 
ent classes  in  this  system.  They  comprise  salts  of  the  alkalies,  K,  Na 
and  NH4,  and  of  silver.  Of  these  only  three  minerals  are  of  importance, 
viz.:  sylvite,  halite  and  cerargyrite. 

Halite  (NaCl) 

Halite,  or  common  salt,  is  the  best  known  and  most  abundant  of  the 
native  chlorides.  It  is  a  colorless,  transparent  mineral  occurring  in 
crystals,  and  in  granular  and  compact  masses. 

Pure  halite  consists  of  39.4  per  cent  CI  and  60.6  per  cent  Na.  The 
mineral  usually  contains  as  impurities  clay,  sulphates  and  organic 
substances.  The  several  analyses  quoted  below  indicate  the  nature  of 
the  commonest  impurities  and  their  abundance  in  typical  specimens. 

NaCl      CaCl     MgCl      CaS04      Na2S04     Mg2S04  Clay     H20 

23       3° 

2.00       .70 

■33 


*•'  97  •  35       ....        .... 

I. Ol 

•43 

II.  90 . 3         ....        .... 

S°° 

2.00 

III.  98.88         tr          tr 

•79 

• .  •  • 

I.  Stassfurt,  Germany. 

II.  Vic,  France. 

III.  Petit  Arise,  La. 

....  •*.. 


The  crystallization  of  halite  is  isometric  (hexoctahedral  class),  the 
principal  forms  being  00  O  00  (100),  O(in)  and  ooO(no).    Often  the 

134 


CHLORIDES,  FLUORIDES,  ETC.  135 

faces  of  the  forms  are  hollowed  or  depressed  giving  rise  to  what  are  called 
"  hopper  crystals  "  (Fig.  59).  The  mineral  occurs  also  in  coarse,  gran- 
ular aggregates,  in  lamellar  and  fibrous  masses  and  in  stalactites. 

Its  cleavage  is  perfect  parallel  to  00  O  00  (100).  Its  fracture  is  con- 
choidal.  Its  hardness  is  2-2.5  and  density  about  2.17.  Halite,  when 
pure,  is  colorless,  but  the  impurities  present  often  color  it  red,  gray, 
yellow  or  blue.  The  bright  blue  mottlings  observed  in 
many  specimens  are  thought  to  be  due  to  the  presence 
of  colloidal  sodium.  The  mineral  is  transparent  or 
translucent  and  its  luster  is  vitreous.  Its  streak  is 
colorless.  Its  saline  taste  is  well  known.  It  is 
diathermous  and  is  a  nonconductor  of  electricity.  pIG  SQ.— Hopper- 
The  mineral  is  plastic  under  pressure;  and  its  plasticity  Shaped  Cube  of 
increases  with  the  temperature.  Its  index  of  refraction  Halite, 
for  sodium  light,  n— 1.5442. 

In  the  closed  tube  halite  fuses  and  often  it  decrepitates.  When 
heated  before  the  blowpipe  it  fuses  (at  7760)  and  colors  the  flame  yellow. 
The  chlorine  reaction  is  easily  obtained  by  adding  a  small  particle  of  the 
mineral  to  a  microcosmic  salt  bead  that  has  been  saturated  with  copper 
oxide.  This,  when  heated  before  the  blowpipe,  colors  the  flame  a  bril- 
liant blue.  The  mineral  easily  dissolves  in  water,  and  its  solution  yields 
an  abundant  white  precipitate  with  silver  nitrate. 

The  solubility  of  halite  is  accountable  for  a  large  number  of 
pseudomorphs.  The  crystals  embedded  in  clays  are  gradually  dissolved, 
leaving  a  mold  that  may  be  filled  by  other  substances,  which  thus 
become  pseudomorphs. 

Syntheses. — Crystals  of  halite  have  been  produced  by  sublimation 
from  the  gases  of  furnaces,  and  by  crystallization  from  solution  contain- 
ing sodium  chloride. 

Occurrence  and  Origin. — Salt  occurs  most  abundantly  in  the  water  of 
the  ocean,  of  certain  salt  lakes,  of  brines  buried  deep  within  the  rocks  in 
some  places,  and  as  beds  interstratified  with  sedimentary  rocks.  In  the 
latter  case  it  is  associated  with  sylvite  (KC1),  anhydrite  (CaS04),  gypsum 
(CaS04-2H20),  etc.,  which,  like  the  halite,  are  believed  to  have  been 
formed  by  the  drying  up  of  salt  lakes  or  of  portions  of  the  ocean  that 
were  cut  off  from  the  main  body  of  water,  since  the  order  of  occurrence 
of  the  various  beds  is  the  same  as  the  order. of  deposition  of  the  corre- 
sponding salts  when  precipitated  by  the  evaporation  of  sea  water  at 
varying  temperatures.     (Comp.  pp.  22,  23.) 

Below  are  given  figures  showing  the  composition  of  the  salts  in  the 
water  of  the  ocean,  of  Great  Salt  Lake,  and  of  the  Syracuse,  N.  Y.,  and 


136  DESCRIPTIVE  MINERALOGY 

Michigan  artificial  brines  (produced  by  forcing  water  to  the  buried  rock 
salt). 

NaCl  CaCl2  MgCl2  NaBr  KC1  Na2S04  K2S04  CaS04  MgS04 

I.  77.07     ....     7.86  1.30    3.89      4.63      5.29 

II.  7957     ....  10.00  6.25      3.60        .58 

III.  9597       .90      .69  2  54 

IV.  91.95    3 .19    2  48  2*39 

I.  Atlantic  Ocean. 
II.  Great  Salt  Lake. 

III.  New  York  brines. 

IV.  Michigan  brines. 

Localities. — The  principal  mines  of  halite,  or  rock  salt,  are  at  Wie- 
lic2ka,  Poland;  Hall,  Tyrol;  Stassfurt,  Germany,  where  fine  crystals 
are  found;  the  Valley  of  Cardova,  Spain;  in  Cheshire,  England  and  in 
the  Punjab  region  of  India.  At  Petit  Anse  in  Louisiana,  in  the  vicinity 
of  Syracuse,  N.  Y.,  and  in  the  lower  peninsula  of  Michigan  thick  beds 
of  the  salt  are  buried  in  the  rocks  far  beneath  the  surface.  Much  of  the 
salt  is  comparatively  pure  and  needs  only  to  be  crushed  to  become  usable. 
In  most  cases,  however,  it  is  contaminated  with  clay  and  other  sub- 
stances. In  these  cases  it  must  be  dissolved  in  water  and  recrystallized 
before  it  is  sufficiently  pure  for  commercial  uses. 

The  best  known  deposits  are  at  Stassfurt  where  there  is  a  great  thick- 
ness of  alternating  layers  of  halite,  sylvite  (KC1),  anhydrite,  gypsum, 
kieserite  (MgS04-H20)  and  various  double  chlorides  and  sulphates  of 
potassium  and  magnesium.  Although  the  halite  is  in  far  greater  quan- 
tity than  the  other  salts,  nevertheless,  the  deposit  owes  most  of  its  value 
to  the  latter,  especially  the  potassium  salts  (comp.  pp.  137,  142). 

Uses. — Besides  its  use  in  curing  meat  and  fish,  salt  is  employed  in 
glazing  pottery,  in  enameling,  in  metallurgical  processes,  for  clearing 
oleomargarine,  making  butter  and  in  the  more  familiar  household  oper- 
ations.   It  is  also  the  chief  source  of  sodium  compounds. 

Production. — Most  of  the  salt  produced  in  the  United  States  is  ob- 
tained directly  from  rock  salt  layers  by  mining  or  by  a  process  of  solu- 
tion, in  which  water  is  forced  down  into  the  buried  deposit  and  then  to 
the  surface  as  brine,  which  is  later  evaporated  by  solar  or  by  artificial 
heat.  In  the  district  of  Syracuse,  N.  Y.,  salt  occurs  in  thick  lenses 
interbedded  with  soft  shales.  In  eastern  Michigan  and  in  Kansas  salt 
is  obtained  from  buried  beds  of  rock  salt,  and  in  Louisiana  from  great 
dome-like  plugs  covered  by  sand,  clay  and  gravel.  Some  of  the  masses 
in  this  State  are  1,756  ft.  thick. 


CHLORIDES,  FLUORIDES,  ETC.  137 

The  salt  production  of  the  United  States  for  191 2  amounted  to  33,- 
324,000  barrels  of  280  lb.  each,  valued  at  $9,402,772.  Of  this  quantity 
7,091,000  barrels  were  rock  salt. 

The  imports  of  all  grades  of  salt  during  the  same  time  were  about 
1,000,000  barrels  and  the  exports  about  440,000  barrels. 

Sylvite  (KC1) 

Sylvite  is  isometric,  like  halite,  but  the  etched  figures  that  may  be 
produced  on  the  faces  of  its  crystals  indicate  a  gyroidal  symmetry  (pen- 
tagonal icositetrahedral  class).  The  habit  of  the  crystals  is  cubic  with 
0(i  11)  and  00  O  00  (100)  predominating. 

Pure  sylvite  contains  47.6  per  cent  CI  and  52.4  per  cent  K,  but  the 
mineral  usually  contains  some  NaCl  and  often  some  of  the  alkaline  sul- 
phates. 

The  physical  properties  of  sylvite  are  like  those  of  halite,  except  that 
its  hardness  is  2  and  the  density  1.99.  Its  melting  temperatuie  is  7380 
and  n  for  sodium  light  =  1.4903. 

When  heated  before  the  blowpipe  the  mineral  imparts  a  violet  tinge 
to  the  flame,  which  can  be  detected  when  masked  by  the  yellow  flame  of 
sodium  by  viewing  it  through  blue  glass.  Otherwise  sylvite  and  halite 
react  similarly. 

Halite  and  sylvite  are  distinguished  from  other  soluble  minerals  by 
the  reaction  with  the  bead  saturated  with  copper  oxide,  and  from  one 
another  by  the  color  imparted  to  the  blowpipe  flame. 

Synthesis. — Sylvite  crystals  have  been  made  by  methods  analogous 
to  those  employed  in  syntheses  of  halite  crystals. 

Occurrence. — Sylvite  occurs  associated  with  halite,  but  in  distinct 
beds,  at  Stassfurt,  Germany,  and  at  Kalusz.  Galicia.  It  has  also  been 
found,  together  with  the  sodium  compound,  incrusting  the  lavas  of 
Vesuvius. 

Uses. — Sylvite  is  an  important  source  of  potassium  salts,  large  quan- 
tities of  which  are  used  in  the  manufacture  of  fertilizers. 

CERARGYRITE  GROUP 

The  cerargyrite  group  comprises  the  chloride,  bromide  and  iodide  of 
silver.  The  first  two  exist  as  the  minerals  cerargyrite  and  bromargyrite, 
both  of  which  crystallize  in  the  isometric  system.  The  isometric  Agl 
exists  only  above  1460;  below  this  temperature  the  iodide  is  hexagonal. 
The  jexhagonal  modification  occurs  as  the  mineral  iodyrite,  which,  of 
course,  is  not  regarded  as  a  member  of  the  cerargyrite  group. 


138  DESCRIPTIVE  MINERALOGY 

Cerargyrite  (AgCl) 

Cerargyrite,  or  horn  silver,  is  an  important  silver  ore.  It  is  usually 
associated  with  other  silver  compounds,  the  mixture  being  mined  and 
smelted  without  separation  of  the  components.  It  is  usually  recog- 
nizable by  its  waxy,  massive  character. 

Silver  chloride  consists  of  24.7  per  cent  chlorine  and  75.3  per  cent 
silver,  but  cerargyrite  often  contains,  in  addition  to  its  essential  con- 
stituents, some  mercury,  bromine  and  occasionally  some  iodine.  Crystals 
are  rare.  They  are  isometric  (hexoctahedral  class),  with  a  cubical  habit, 
their  predominant  forms  being  00  O  00  (100),  00  O(no),  O(in),  20(221) 
and  202(211).  Twins  sometimes  occur  with  O(in)  the  twinning  face. 
The  mineral  is  sometimes  found  massive,  embedded  among  other  min- 
erals, but  is  more  frequently  in  crusts  covering  other  substances. 

The  fracture  of  cerargyrite  is  conchoidal.  The  mineral  is  sectile. 
Its  hardness  is  1-1.5  an^  density  about  5.5.  Its  color  is  grayish,  white 
or  yellow,  sometimes  colorless.  On  exposure  to  light  it  turns  violet- 
brown.  It  is  transparent  to  translucent  and  its  streak  is  white.  It  is  a 
very  poor  conductor  of  electricity.  Like  halite  it  is  diathermous.  n  for 
sodium  light  =  2.071. 

In  the  closed  tube  cerargyrite  fuses  without  decomposition.  On 
charcoal  it  yields  a  metallic  globule  of  silver,  and  when  heated  with  oxide 
of  copper  in  the  blowpipe  flame  it  gives  the  chlorine  reaction.  The  min- 
eral is  insoluble  in  water  and  in  nitric  acid  but  is  soluble  in  ammonia,  and 
potassium  cyanide.  When  a  particle  of  the  mineral  is  placed  on  a 
sheet  of  zinc  and  moistened  with  a  drop  of  water,  it  swells,  turns  black 
and  is  finally  reduced  to  metallic  silver,  which,  when  rubbed  by  a  knife 
blade,  exhibits  the  white  luster  of  the  metal. 

Cerargyrite  is  easily  distinguished  from  all  other  minerals,  except 
the  comparatively  rare  bromide  and  iodide,  by  its  physical  properties  and 
by  the  metallic  globule  which  it  yields  on  charcoal. 

Syntheses. — Crystals  of  cerargyrite  have  been  obtained  by  the  rapid 
evaporation  of  ammoniacal  solutions  of  silver  chloride,  and  by  the  cooling 
of  solutions  of  the  chloride  in  molten  silver  iodide. 

Occurrence. — The  mineral  occurs  in  the  upper  (oxidized)  portions  of 
veins  of  argentiferous  minerals,  where  it  is  associated  with  native  silver 
and  oxidized  products  of  various  kinds. 

Localities. — The  most  important  localities  of  cerargyrite  are  in  Peru, 
Chile,  Honduras  and  Mexico,  where  it  is  associated  with  native  silver. 
It  is  also  found  near  Leadville,  Colo,;  near  Austin,  in  the  Comstock 
lode,  Nev.,  and  at  the  Poorman  Mine,  and  in  other  mines  in  Idaho 


CHLORIDES,  FLUORIDES,  ETC.  139 

and  at  several  places  in  Utah.     Good  crystals  occur  in  the  Poorman 

Extraction. — When  a  silver  ore  consists  essentially  of  cerargyrite  the 
metal  may  be  extracted  by  amalgamation.  Ores  containing  compara- 
tively small  quantities  of  cerargyrite  are  smelted. 

Production. — The  quantity  of  cerargyrite  mined  cannot  be  safely 
estimated.  As  has  been  stated,  it  is  usually  wrought  with  other  silver 
ores. 

THE  FLUORIDES 

The  fluorides  are  salts  of  hydrofluoric  acid.  There  are  several 
known  to  occur  as  minerals,  but  only  two,  the  fluoride  of  calcium  and 


Pig  to.— Group  of  Fluorite  Crystals  from  Wear  dale,  Co.,  Durham,  England.     (Foots 
Mineral  Company.) 

the  double  fluorides  of  sodium  and  aluminium  are  of  sufficient  impor- 
tance to  merit  description  here. 

Fluorite  (CaF2) 
Fluorite,  or  fluorspar,  is  the  principal  source  of  fluorine.    It  is  usually 
a  transparent  mineral  that  is  characterized  by  its  fine  color  and  its  hand- 


140 


DESCRIPTIVE  MINERALOGY 


some  crystals  (Fig.  60).  Perhaps  there  is  no  other  mineral  known  that 
can  approach  it  in  the  beauty  of  its  crystal  groups.  The  uncrystallized 
fluorite  may  be  massive,  granular  or  fibrous. 

Fluorite  is  a  compound  of  Ca  and  F  in  the  proportion  of  48.9  per  cent 
F  and  51.1  per  cent  Ca.  Chlorine  is  occasionally  present  in  minute 
quantities,  and  Si02,  AI2O3  and  Fe2<I>3  are  always  present.  A  sample  of 
commercially  prepared  fluorite  from  Marion,  Ky.,  gave: 


CaF2 

Si02 

Al203+Fe203 

CaC03 

MgO 

94.72 

1 .22 

.98 

1.82 

.68 

The  crystallization  is  isometric  (hexoctrahedral  class),  and  inter- 
penetration  twins  are   frequent.     The  principal   forms   observed  are 


0 


^ 


Fig.  61. 


Fig.  62. 


Fig.  61. — Crystal  of  Fluorite  witji  °o  O  «> ,  100  (a)  and  °o  O2,  210  (e). 
Fig.  62. — Intcrpenetration  Cubes  of  Fluorite,  Twinned  about  O(iu). 


O(iii),  ooOoo(ioo),  0002(210)  and  402(421)  (Fig.  61),  but  some  crys- 
tals are  highly  modified,  as  many  as  58  forms  having  been  identified  upon 
the  species.  The  twins,  with  O(in)  the  twinning  plane,  are  usually 
interpenetration  cubes,  or  cubes  modified  on  the  corners  by  the  octa- 
hedrons (Fig.  62).  The  mineral  occurs  also  in  granular,  fibrous  and 
earthy  masses. 

The  cleavage  of  fluorite  is  perfect  parallel  toO(in).  The  mineral 
is  brittle,  its  fracture  is  uneven  or  conchoidal,  its  hardness  is  4  and  its 
density  about  3.2.  It  melts  at  13870.  Its  color  is  some  shade  of  yel- 
low, white,  red,  green,  blue  cr  purple,  its  luster  vitreous,  and  its  streak 
is  white.  Many  specimens  are  transparent,  some  are  only  translucent. 
Most  specimens  phosphoresce  upon  heating.  A  variety  that  exhibits  a 
green  phosphoresence  is  known  as  chlorophane.  The  index  of  refraction 
for  sodium  light  is  1.43385  at  200.  The  mineral  is  a  nonconductor  of 
electricity. 

The  color  of  the  brightly  tinted  varieties  was  formerly  thought  to  be 
due  to  the  presence  of  minute  traces  of  organic  substance  since  it  is  lost 


CHLORIDES,   FLUORIDES,  ETC.  141 

or  changed  when  the  mineral  is  heated,  but  recent  observations  of  the 
effect  of  radium  emanations  upon  light-colored  specimens  indicate  a 
deepening  of  their  color  by  an  increase  in  the  depth  of  the  blue  tints. 
This  suggests  that  the  coloring  matter  is  combined  with  the  CaF2.  It 
may  be  a  colloidal  substance. 

In  the  closed  tube  fluorite  decrepitates  and  phosphoresces.  When 
heated  on  charcoal  it  fuses,  colors  the  flame  yellowish  red  and  yields  an 
enamel-like  residue  which  reacts  alkaline  to  litmus  paper.  Its  powder 
treated  with  sulphuric  acid  yields  hydrofluoric  acid  gas  which  etches 
glass.  The  same  effect  is  produced  when  the  powdered  mineral  is  fused 
with  four  times  its  volume  of  acid  potassium  sulphate  (HKSO4)  in  a 
glass  tube.  The  walls  of  the  tube  near  the  mixture  become  etched  as 
though  acted  upon  by  a  sand  blast. 

Fluorite  is  easily  distinguished  by  its  cleavage  and  hardness  from 
most  other  minerals.  It  is  also  characterized  by  the  possession  of 
fluorine  for  which  it  gives  clear  reactions. 

Syntheses. — Crystals  are  produced  upon  the  cooling  of  a  molten  mix- 
ture of  CaF2  and  the  chlorides  of  the  alkalies,  and  by  heating  amorphous 
CaF2  with  an  alkaline  carbonate  and  a  little  HC1  in  a  closed  tube  at  250°. 
Occurrence,  Localities  and  Origin. — The  mineral  occurs  in  beds,  in 
veins,  often  as  the  gangue  of  metallic  ores  and  as  crystals  on  the  walls 
of  cavities  in  certain  rocks.  It  is  the  gangue  of  the  lead  veins  of  northern 
England  and  elsewhere.  Handsome  crystallized  specimens  come  from 
Cumberland  and  Derbyshire,  England;  Kongsberg,  Norway;  Cornwall, 
Wales,  and  from  the  mines  of  Saxony.  In  the  United  States  the  mineral 
forms  veins  on  Long  Island;  in  Blue  Hill  Bay,  Maine;  at  Putney,  in 
Vermont;  at  Plymouth,  Conn.;  at  Lockport  and  Macomb,  in  New 
York;  at  Amelia  Court  House,  Va.,  and  abundantly  in  southeastern 
Illinois  and  the  neighboring  portion  of  Kentucky,  where  it  occurs  asso- 
ciated with  zinc  and  lead  ores.  These  last-named  localities,  the  neigh- 
borhood of  Mabon  Harbor,  Nova  Scotia,  and  Thunder  Bay,  Lake 
Superior,  afford  excellent  crystal  groups.  In  nature  fluorite  has  been 
apparently  produced  both  by  .crystallization  from  solutions  and  by 
pneumatolytic  processes. 

Since  fluorite  is  soluble  in  alkaline  waters,  its  place  in  the  rocks  is  often 
occupied  by  calcite,  quartz  or  other  minerals  that  pseudomorph  it. 

Uses. — The  mineral  is  used  extensively  as  a  flux  in  smelting  iron  and 
other  ores,  in  the  manufacture  of  opalescent  glass,  and  of  the  enamel 
coating  used  on  cooking  utensils,  etc.  It  is  also  used  in  the  manufacture 
of  hydrofluoric  acid,  which,  in  turn,  is  employed  in  etching  glass.  The 
brighter  colored  varieties  are  employed  as  material  for  vases  and  the 


142  DESCRIPTIVE  MINERALOGY 

transparent,  colorless  kinds  are  ground  into  lenses  for  optical  instruments. 
The  mineral  is  also  cut  into  cheap  gems,  known  according  to  color,  as 
false  topaz,  false  amethyst,  etc.  Except  when  used  for  making  lenses  or 
as  a  precious  stone,  fluorite  is  prepared  for  shipment  by  crushing,  wash- 
ing and  screening.    A  portion  is  ground. 

Production. — The  fluorite  produced  in  the  United  States  is  obtained 
mainly  from  Illinois  and  Kentucky,  though  small  quantities  are  mined 
in  Colorado,  New  Mexico  and  New  Hampshire.  The  production  in 
1912  amounted  to  116,545  tons,  valued  at  $769,163.  Of  this,  114,410 
tons  came  from  Illinois  and  Kentucky.  The  imports  were  26,176  tons, 
valued  at  $71,616. 

THE  DOUBLE  CHLORIDES  AND  DOUBLE  FLUORIDES 

These  double  salts  are  apparently  molecular  compounds,  in  which 
usually  two  chlorides  or  two  fluorides  combine,  as  in  AlF3+3NaF. 
Moreover,  one  of  the  members  of  the  combination  of  chlorides  is  nearly 
always  either  the  sodium  or  the  potassium  chloride.  The  law  of  this 
combination  is  expressed  by  Professor  Remsen  in  these  words:  "  The 
number  of  molecules  of  potassium  or  sodium  chloride  which  combine 
with  another  chloride  is  limited  by  the  number  of  chlorine  atoms  con- 
tained in  the  other  chloride."  Thus,  if  NaCl  makes  double  salts  with 
MCI2,  in  which  M  represents  any  bivalent  element,  only  two  are  possible, 
viz:  MCb+NaCl  and  MCl2+2NaCl.  With  MCI3  three  double  salts 
with  sodium  may  be  formed,  etc.  These  double  salts  are  not  regarded 
as  true  molecular  compounds,  but  they  are  looked  upon  as  compounds 
in  which  CI  and  F  are  bivalent  like  oxygen. 

Carnallite  (KMgCk  6H20) 

Carnallite  may  be  regarded  as  a  hydrated  double  chloride  of  the 
composition  MgCfe  •  KC1  •  6H2O  with  14.1  per  cent  K,  8.7  per  cent  Mg, 
38.3  per  cent  CI  and  39.0  per  cent  H2O.  It  occurs  in  distinct  crys- 
tals but  more  frequently  in  massive  granular  aggregates. 

Its  crystallization  is  orthorhombic  (bipyramidal  class),  but  the  habit 
of  its  crystals  is  usually  hexagonal  because  of  the  nearly  equal  develop- 
ment of  pyramids  and  brachy  domes.  Its  axial  ratio  is  .5891  :  1  :  1.3759. 
Crystals  are  commonly  bounded  by  00  P(no),  P(in),  ^P(ii2),  $P(ii3), 
00  P  06  (010),  2P  06  (021),  P  06  (on),  I P  06  (023),  oP(ooi),  and  P  00  (101). 
The  angle  noAi^o=6i°  2o§'. 

Carnallite  is  colorless  lo  milky  white,  transparent  or  translucent, 
and  has  a  fatty  luster.     Many  varieties  appear  red  in  the  hand  specimens 


CHLORIDES,  FLUORIDES,  ETC.  143 

because  of  the  inclusion  of  numerous  small  plates  of  hematite  or  goethite, 
or  yellow  because  of  inclusions  of  yellow  liquids  or  tiny  crystals.  The 
mineral  has  a  hardness  of  1-3,  and  a  density  of  1.60.  It  possesses  no 
cleavage  but  has  a  conchoidal  fracture.  It  is  not  an  electrical  conductor. 
It  is  deliquescent  and  has  a  bitter  taste.  Its  indices  of  refraction  for 
sodium  light  are  a=  1.467,  0=  1.475,  T=  I-494« 

Before  the  blowpipe  carnallite  fuses  easily.  In  the  closed  tube  it 
becomes  turbid  and  gives  off  much  water,  which  is  frequently  accom- 
panied by  the  odor  of  chlorine.  It  melts  in  its  own  water  of  crystalliza- 
tion. When  evaporated  to  dryness  and  heated  by  the  blowpipe  flame 
a  white  mass  results  which  is  strongly  alkaline.  The  mineral  dissolves 
in  water,  forming  a  solution  which  reacts  for  Mg,  K  and  CI. 

Carnallite  is  easily  recognized  by  its  solubility,  its  bitter  taste  and  the 
reaction  for  chlorine. 

Synthesis. — The  mineral  separates  in  measurable  crystals  from  a  solu- 
tion of  MgCl2  and  KC1. 

Occurrence  and  Origin. — Carnallite  occurs  in  beds  associated  with 
sylvite,  halite,  kieserite  (p.  246),  and  other  salts  that  have  been  pre- 
cipitated by  the  evaporation  of  sea  water  or  the  water  of  salt  lakes. 

Localities. — It  is  found  in  large  quantity  at  Stassfuxt,  Germany;  at 
Kalusz,  in  Galicia  and  near  Maman,  in  Persia. 

Uses. — Carnallite  is  used  as  a  fertilizer  and  as  a  source  of  potash 
salts. 

Cryolite  (NasAlF*) 

Cryolite  usually  occurs  as  a  fine-grained  granular  white  mass  in 
which  are  often  embedded  crystals  of  light  brown  iron  carbonate  (sider- 
ite).  The  formula  given  above  demands  54.4  per  cent  F,  12.8  per  cent 
Al  and  32.8  per  cent  Na.  Analyses  of  pure  white  specimens  correspond 
veiy  closely  to  this. 

The  mineral  is  monoclinic  (prismatic  class),  but  crystals  are  exceed- 
ingly rare  and  when  found  they  have  a  cubical  habit.  Their  axial  ratio 
is  a  :  b  :  ^=.9662  :  1  :  1.3882.  £=89°  49'.  The  principal  forms  are 
ooP(no),  oP(ooi),  Poo(oTo),  —Poo  (010)  and  Poo  (100),  thus  re- 
sembling the  combination  of  the  cube  and  octahedron.  Twins  are  com- 
mon, with  00  P(no)  the  twinning  plane. 

The  cleavage  of  cryolite  is  perfect  parallel  to  oP(coi).  Its  fractuie 
is  uneven.  Hardness  is  2.5  and  density  about  3.  Its  color  is  snow-white 
inclining  to  red  and  brown.  Its  luster  is  vitreous  or  greasy  and  the 
mineral  is  translucent  to  transparent.  Because  of  its  low  index  of 
refraction,  massive  specimens  suggest  masses  of  wet  snow.    The  re- 


144  DESCRIPTIVE  MINERALOGY 

fractive  index  0  f or  sodium  light  is  1.364.  It  is  a  nonconductor  of 
electricity. 

Cryolite  is  very  easily  fusible,  small  pieces  melting  even  at  the  low 
temperature  of  a  candle  flame.  The  mineral  is  soluble  in  sulphuric  acid 
with  the  evolution  of  HF.  When  fused  in  the  closed  tube  with  KHSO4 
it  yields  hydrofluoric  acid,  and  when  fused  on  charcoal  fluorine  is  evolved. 
The  residue  treated  with  Co(N(>3)2  and  heated  gives  the  color  reaction 
forAl. 

By  the  aid  of  its  reactions  with  sulphuric  acid,  its  fusibility  and  its 
physical  properties  cryolite  is  easily  distinguished  from  fluorite,  .which  it 
most  resembles,  and  from  all  other  minerals. 

Occurrence,  Localities  and  Origin. — The  occurrences  of  cryolite  are 
very  few.  It  has  been  found  in  smaM  quantities  near  Miask  in  the 
Ilmen  Mts.,  Russia;  near  Pike's  Peak,  Colo.,  and  in  the  Yellowstone 
National  Park.  Its  principal  occurrence  is  in  a  great  pegmatitic  vein 
cutting  granite  near  Ivigtut,  Greenland,  whence  all  the  mineral  used 
in  the  arts  is  obtained.  The  associates  of  the  cryolite  at  this  place  are 
siderite,  galena,  chalcopyrite,  p>rite,  fluorite,  topaz  and  a  few  rare 
minerals.  The  vein  is  said  to  be  intrusive  into  the  granite.  It  is 
believed  to  be  a  magmatic  concentration. 

Uses. — Cryolite  was  formerly  employed  principally  in  the  manufac- 
ture of  alum  and  of  salts  of  sodium.  At  present  it  is  used  as  a  flux  in 
the  electrolytic  production  of  aluminium,  and  is  employed  in  the  man- 
ufacture of  white  porcelain-like  glass,  and  in  the  process  of  enameling 
iron.  The  mineral  is  quarried  in  Greenland  and  imported  into  the 
United  States  to  the  extent  of  about  2,500  tons  annually.  Its  value  is 
about  $25  per  ton. 

THE  OXYCHLORIDES 

The  oxychlorides  are  combinations  of  hydroxides  and  chlorides. 

Some  of  them  are  "  double  salts  "  in  the  sense  in  which  this  word  is 

explained   above.      Atacamite   is   a   combination   of   the   oxychloride 

HO\ 
Cu(OH)Cl  with  the  hydroxide  Cu(OH)2,  or         >Cu-Cu(OH)2. 

CV 

Atacamite  (Cu(OH)ClCu(OH)2) 

Atacamite  is  especially  abundant  in  South  America.  The  mineral 
is  usually  found  in  crystalline,  fibrous  or  granular  aggregates  of  a  bright 
green  color. 

Analyses  of  specimens  from  Australia  and  from  Atacama,  Chile,  yield: 


Cu 

CuO 

H20 

Total 

14.67 

56.64 

12.02 

99-77 

14.16 

SS- 70 

14-31 

100.00 

CHLORIDES,  FLUORIDES,  ETC.  146 

CI 

Australia 16.44 

Atacama,  Chile *5«83 

The  formula  requires  16.6  per  cent  CI,  14.9  per  cent  Cu,  55.8  per  cent 
CuO  and  12.7  per  cent  H2O. 

The  crystallization  of  atacamite  is  orthorhombic  (bipyramidal  class), 
with  a:  b:  ^=.6613  :  1  :  .7529.  Its  crystals  are  usually  slender  prisms 
bounded  by  ooP(no),  <»P2(i2o),  00  P  06  (010),  Poo  (on),  oP(ooi) 
and  P(ni),  or  tabular  forms  flattened  in  the  plane  of  the  macropinacoid 
00 Poo  (100).  Twins  are  common,  with  the  twinning  plane  ooP(no). 
The  cleavage  of  atacamite  is  perfect  parallel  to  00  P  06  (010).  Its 
fracture  is  conchoidal.  Its  hardness  is  3-3.5  and  density  about  3.76. 
Pure  atacamite  is  of  some  shade  of  green,  varying  between  bright  shades 
and  emerald.  Its  aggregates  often  contain  red  or  brown  streaks  or 
grains  due  to  the  admixture  of  copper  oxides.  It  is  transparent  to  trans- 
lucent. The  streak  of  the  mineral  is  apple-green.  It  is  a  nonconductor 
of  electricity.  Its  indices  of  refraction  for  green  light  are:  0=1.831, 
0=  1.861,7=  1.880. 

In  the  closed  tube  atacamite  gives  off  much  water  with  an  acid  reac- 
tion, and  yields  a  gray  sublimate.  In  the  oxidizing  flame  it  fuses  and 
tinges  the  flame  azure  blue  (reaction  for  copper  chloride).  It  is  easily 
reduced  to  a  globule  of  copper  on  charcoal  and  is  easily  soluble  in  acids. 
Atacamite  is  readily  distinguished  from  garnierite,  malachite  and 
other  green  minerals  by  its  solubility  in  acids  without  effervescence  and 
by  the  azure  blue  color  it  imparts  to  the  flame. 

Synthesis. — Crystals  have  been  produced  by  heating  cuprous  oxide 
(CU2O)  with  a  solution  of  FeCl3,  in  a  closed  tube  at  2500. 

Occurrence,  Localities  and  Origin. — The  mineral  is  most  abundant 
along  the  west  side  of  the  Andes  Mountains  in  Chile  and  Bolivia.  It 
occurs  also  in  South  Australia;  in  India;  at  Ambriz,  on  the  west  coast  of 
Africa;  in  southern  Spain;  in  Cornwall,  where  it  forms  stalactite  tubes; 
in  southern  California,  and  near  Jerome,  Arizona.  It  is  formed  as  the 
result  of  the  alteration  of  other  copper  compounds,  and  is  found  most 
abundantly  in  the  upper  portions  of  copper  veins.  Atacamite  changes 
on  exposure  to  the  weather  into  the  carbonate,  malachite,  and  the  sili- 
cate, chrysocolla. 

Uses. — The  mineral  is  an  important  ore  of  copper,  but  it  is  mined 
with  other  compounds  and  consequently  no  records  of  the  quantity 
obtained  are  available. 


CHAPTER  VII 

THE  OXIDES 

The  oxides  (except  water)  and  the  hydroxides  may  be  regarded  as 
derivatives  of  water,  the  hydrogen  being  replaced  wholly  or  in  part 
by  a  metal.  When  only  part  of  the  hydrogen  is  replaced  an  hydroxide 
results;  when  all  of  the  hydrogen  is  replaced  an  oxide  results.  Thus, 
sodium  hydroxide,  NaHO,  may  be  looked  upon  as  H2O,  in  which  Na  has 
replaced  one  atom  of  H,  and  sodium  oxide,  Na2<3,  as  H2O  in  which  both 
hydrogen  atoms  have  been  replaced  by  this  element.  Ferric  oxide  and 
ferric  hydroxide  bear  these  relations  to  water: 

H-O— H  O 

H 


H-— 0 — H,    Fe — 0 — Fe,   ferric  oxide,     H — O — Fe,  ferric  hydroxide. 

,/  Fe203        H-o/  Fe(OH)3 


H— O— H  O 


The  oxides  constitute  a  very  important,  though  not  a  large,  class  of 
minerals.  Some  of  them  are  among  the  most  abundant  of  all  minerals. 
They  are  separated  into  the  following  groups:  Monoxides,  sesqui- 
oxides,  dioxides  and  higher  oxides. 

THE  MONOXIDES 

Ice  (H20) 

The  properties  of  ice  are  so  well  known  that  they  need  no  special 
description  in  this  place.  The  mineral  is  never  pure,  since  it  contains, 
in  all  cases,  admixtures  of  various  soluble  salts.  Its  crystallization  is 
hexagonal  and  probably  trigonal  and  hemimorphic  (di trigonal  pyram- 
idal class).  Crystals  are  often  prismatic,  as  when  ice  .forms  the  cover- 
ing of  water  surfaces,  or  the  bodies  known  as  hailstones.  In  the  form 
of  snow  the  crystals  are  often  stellate,  or  skeleton  crystals,  and  sometimes 

146 


OXIDES  147 

hollow  prisms.    The  principal  forms  observed  on  ice  crystals  are  oP(oooi) 
ooP(ioTo),  iPCioTa),  P(ioTi)  and-jPUo+O  (Fig.  63). 

The  hardness  of  ice  is  about  1.5  and  its  density  .9181.  It  is  trans- 
parent and  colorless  except  in  large  masses  when  it  appears  bluish.  Its 
fracture  is  conchoidal.    It  possesses  no  distinct  cleavage.    Its  fusing 


Fig.  63. — Photographs  of  Snow  Crystals,   Magnified  about  15  Diameters.     (After 

BtnUey  and  Perkins^ 

point  is  o"  and  boiling  point  ico°.     It  is  a  poor  conductor  of  electricity. 
Its  indices  of  refraction  for  sodium  light  at  8°  are:  «™  1.3090,  «™  1-3133. 

COPPER  OXIDES 

There  are  two  oxides  of  copper,  the  red  cuprous  oxide  (Cu;0)  and 
the  black  cupric  oxide  (CuO).  Both  are  used  as  ores,  the  former  being 
much  more  important  a  source  of  the  metal  than  the  latter. 

Cuprite  (C113O) 

Cuprite  occurs  in  crystals,  in  granular  and  earthy  aggregates  and 
massive.  The  mineral  is  usually  reddish  brown  or  red  and  thus  is  easily 
distinguished  from  most  other  minerals.  Its  composition  when  pure  is 
88.8  per  cent  Cu  and  n. 2  per  cent  O. 

In  crystallization  the  mineral  is  isometric,  in  the  gyroidal  hemihedral 
division  of  the  system  (pentagonal  icositetrahedral  class).    Its  pre- 


148  DESCRIPTIVE  MINERALOGY 

dominant  forms  are  ooO°o(ioo),  O(in),  ooO(no),  0002(210), 
202(211),  20(221)  and  30$(32i)>  sometimes  lengthened  out  into 
capillary  crystals,  producing  fibrous  varieties  (var.  chalcotriekite). 

The  cleavage  of  cuprite  is  fairly  distinct  parallel  to  O(in).  Its  frac- 
ture is  uneven  or  conchoidal.  Its  hardness  is  3.5-4  and  density  about  6. 
The  mineral  is  in  some  cases  opaque;  oftener  it  is  translucent  or  even 
transparent  in  very  thin  pieces.  By  reflected  light  its  color  is  red, 
brown  and  occasionally  black.  By  transmitted  light  it  is  crimson.  When 
gently  heated  transparent  varieties  turn  dark  and  become  opaque,  but 
they  reassume  their  original  appearance  upon  cooling.  Its  streak  is 
brownish  red  and  has  a  brilliant  luster.  When  rubbed  it  becomes  yellow 
and  finally  green.  The  luster  of  the  mineral  varies  between  earthy  and 
almost  vitreous.  It  is  a  poor  conductor  of  electricity,  but  its  con- 
ductivity increases  rapidly  with  rising  temperature.  Its  refractive  index 
for  yellow  light=  2.705. 

In  the  blowpipe  flame  cuprite  fuses  and  colors  the  mantle  of  the 
flame  green.  If  moistened  with  hydrochloric  acid  before  heating  the 
flame  becomes  a  brilliant  azure  blue.  On  charcoal  the  mineral  first 
fuses  and  then  is  reduced  to  a  globule  of  metallic  copper.  It  dissolves  in 
strong  hydrochloric  acid,  forming  a  solution  which,  when  cooled  and 
diluted  with  cold  water,  yields  a  white  precipitate  of  cuprous  chloride 

(Cu2Cl2). 

Cuprite  may  easily  be  distinguished  from  other  minerals  possessing 
a  red  streak  by  the  reaction  for  copper — such  as  the  production  of  a 
metal  globule  on  charcoal,  and  the  formation  of  cuprous  chloride  in  con- 
centrated hydrochloric  acid  solutions  by  the  addition  of  water.  More- 
over, the  mineral  is  softer  than  hematite  and  harder  than  reaglar,  cin- 
nabar and  proustite. 

Cuprite  suffers  alteration  very  readily.  It  may  be  reduced  to  native 
copper,  in  which  case  the  copper  pseudomorphs  the  cuprite,  or,  on  ex- 
posure to  the  air  it  may  be  changed  into  the  carbonate,  malachite, 
pseudomorphs  of  which  after  cuprite  are  common. 

Syntheses. — Crystals  of  cuprite  have  frequently  been  observed  on 
copper  utensils  and  coins  that  had  been  buried  for  long  periods  of  time. 
Crystals  have  also  been  obtained  by  long-continued  action  of  NH3  upon 
a  mixture  of  solutions  of  the  sulphates  of  iron  and  copper,  and  by  heating 
a  solution  of  copper  sulphate  and  ammonia  with  iron  wire  in  a  closed  tube. 

Occurrence,  Origin  and  Localities. — Cuprite  often  occurs  as  well 
defined  crystals  embedded  in  certain  sedimentary  rocks  in  the  upper, 
oxidized  portions  of  copper  veins,  and  in  masses  in  the  midst  of  other 
copper  ores,  from  which  it  was  produced  by  oxidation  processes.    It  is 


OXIDES  149 

found  as  crystals  in  Thuringia,  in  Tuscany,  on  the  island  of  Elba,  in 
Cornwall,  Eng.,  at  Chessy,  France;  and  near  Coquimb6,  in  Chile. 
In  Chile,  in  Peru,  and  in  Bolivia  it  exists  in  great  masses. 

In  the  United  States  it  occurs  at  Cornwall,  Lebanon  Co.,  Penn.  It 
is  also  found  associated  with  the  native  copper  on  Keweenaw  Point, 
Mich.;  at  the  copper  mines  in  St.  Genevieve  Co.,  Mo.;  at  Bisbee  and 
at  other  places  in  Arizona.  The  fibrous  variety  known  as  chalcolrichite 
is  beautifully  developed  at  Morenci  in  the  same  State. 

Uses. — Cuprite  is  mined  with  other  copper  compounds  as  an  ore  of 
copper. 

Melaconite,  or  Tenorite  (CuO) 

Melaconite,  or  tenorite,  is  less  common  than  cuprite.  It  usually 
occurs  in  massive  forms  or  in  earthy  masses.  Crystals  are  rare.  Its 
composition  is  79.8  per  cent  Cu  and  20.2  per  cent  O. 

In  crystallization  melaconite  is  triclinic  with  a  monoclinic  habit. 
Its  axial  ratio  is  a :  b  :  c— 1.4902  :  1  :  1.3604  and  /9=99°  32'.  The 
angles  a  and  7  are  both  900,  but  the  optical  properties  of  the  crystals 
proclaim  their  triclinic  symmetry. 

The  mineral  possesses  an  easy  cleavage  parallel  to  oP(ooi).  Its  frac- 
ture is  conchoidal  and  uneven,  its  hardness  3  to  4  and  density  about  6. 
When  it  occurs  in  thin  scales  its  color  is  yellowish  brown  or  iron  gray. 
When  massive  or  pulverulent  it  is  dull  black.  Its  streak  is  black,  chang- 
ing to  green  when  rubbed.  Its  refractive  index  for  red  light  is  2.63. 
It  is  a  nonconductor  of  electricity. 

The  chemical  reactions  of  melaconite  are  precisely  like  those  of  cu- 
prite, with  the  exception  that  the  mineral  is  infusible. 

Melaconite  is  distinguished  from  the  black  minerals  that  contain  no 
copper  by  its  reaction  for  this  metal.  It  is  distinguished  from  covellite 
and  other  dark-colored  sulphides  containing  copper  by  its  failure  to  give 
the  sulphur  reaction. 

Syntheses. — Crystals  of  melaconite  have  been  found  in  the  flues  of 
furnaces  in  which  copper  compounds  and  moist  NaCl  are  being  treated. 
They  have  also  been  obtained  by  the  decomposition  of  CuCb  by  water 
vapor. 

Occurrence,  Localities  and  Origin. — The  mineral  usually  occurs  associ- 
ated with  other  ores  of  copper,  from  which  it  has  been  formed,  in  part 
at  least,  by  decomposition.  It  is  mined  with  these  as  an  ore.  Thin 
scales  are  found  on  the  lava  of  Vesuvius,  where  it  must  have  been  formed 
by  sublimation.    Masses  occur  at  the  copper  mines  of  Ducktown,  Tenn. 


150  DESCRIPTIVE  MINERALOGY 

Zincite  (ZnO) 

Zincite  is  the  only  oxide  of  the  zinc  group  of  elements  known.  It  is 
rarely  found  in  crystals.  It  usually  occurs  in  massive  forms  associated 
with  other  zinc  compounds. 

Pure  zincite  is  a  compound  containing  80.3  per  cent  Zn  and  19.7  per 
cent  O.    Since,  however,  the  mineral  is  frequently  admixed  with  man- 
ganese compounds  it  often  contains  also  some  manganese  and  a  little 
A  iron.    A  specimen  from  Sterling  Hill,  N.  J., 

jm\  gave  98.28  per  cent  ZnO,  6.50  per  cent  MnO 

//I  uV  and  .44  per  cent  Fe203. 

//I    \\\  Natural  crystals  of  zincite  are  very  rare. 

// pi  \\p\  From  a  study  of  artificial  crystals  it  is  known 
/  /  J  P  \\\  ^at  ^e  mhieral  is  hexagonal  and  hemimorphic 
x|     j         T  \  *>|     (dihexagonal  pyramidal  class).    The  principal 

I   J^^f ^-X™]     forms  observed  are    ooP(ioTo),     ooP2(n5o), 

*^-*-^l       w»     g.  \^s>     oP(oooi),  P(ioTi),  P2(ii22)  and  various  other 

Fig.  64.— Zincite  Crystal    pyramids  of  the  1st  and  2d  orders.     Their  habit 

with   00P,    10T0   (m);    js  hemimorphic  with  P(ion)  and  oP(oooi)  at 

P,  ion  (p)  and   oP,     the  oppOSite  en(k  0f  a  short  columnar  crystal 

OOOI      (c).  /T?.  ,        V 

(Fig.  64). 

The  cleavage  of  zincite  is  perfect  parallel  to  oP(oooi).  Its  fracture 
is  conchoidal,  its  hardness  4-4.5  and  density  about  5.8.  Although  color- 
less varieties  are  known,  the  mineral  is  nearly  always  deep  red  or  orange- 
yellow,  due  most  probably  to  the  manganese  present  in  it.  The  streak 
of  the  red  varieties  is  orange- yellow.  Its  indices  of  refraction  are 
about  2.    The  mineral  is  a  conductor  of  electricity. 

When  heated  in  the  closed  tube  the  common  variety  of  zincite 
blackens,  but  it  resumes  its  original  color  on  cooling.  With  the  borax 
bead  it  gives  the  manganese  reaction.  Heated  on  charcoal  it  coats  the 
coal  with  a  white  film,  which,  when  moistened  with  cobalt  solution  and 
heated  again  with  the  oxidizing  flame  of  the  blowpipe,  turns  green.  The 
mineral  dissolves  in  acids. 

When  exposed  to  the  atmosphere  zincite  undergoes  slow  decomposi- 
tion to  zinc  carbonate. 

Syntheses. — Zinc  oxide  crystals  are  frequent  products  of  the  roasting 
of  zinc  ores  in  ovens.  They  have  also  been  produced  by  the  action  of 
zinc  chloride  vapor  upon  lime  and  by  the  action  of  water  upon  zinc 
chloride  at  a  red  heat. 

Occurrence  and  Localities. — The  mineral  occurs  only  in  a  few  places. 
It  is  found  with  other  zinc  and  manganese  minerals  near  Ogdensburg, 


OXIDES  151 

and  at  Franklin  Furnace,  in  Sussex  Co.,  N.  J.,  in  the  form  of  great 
layers  in  marble,  that  are  bent  into  troughs.  The  layers  are  probably 
veins  that  were  filled  from  below  by  emanations  from  a  great  underground 
reservoir  of  igneous  rock. 

Uses. — Most  of  the  zincite  produced  in  the  United  States  is  used  in 
the  manufacture  of  zinc  oxide.  The  ore,  which  consists  of  a  mixture  of 
zincite,  franklinite  (see  p.  199),  and  willemite  (see  p.  306),  is  crushed 
and  separated  into  its  component  parts  by  mechanical  processes.  The 
separated  zincite  is  then  mixed  with  coal  and  roasted.  The  zinc  oxide 
is  volatilized  and  is  caught  in  tubes  composed  of  bagging.  The  willemite 
and  franklinite  are  smelted  to  metallic  zinc  and  the  residues  are  used  in 
the  manufacture  of  spiegeleisen. 

Production. — Formerly  this  mineral,  together  with  the  silicate  found 
associated  with  it  in  New  Jersey,  constituted  the  most  important  source 
of  zinc  in  this  country.  At  present  most  of  the  metal  is  obtained  from 
sphalerite.  Of  the  380,000  tons  of  zinc  in  spelter  and  zinc  compounds 
produced  in  the  United  States  during  191 2  about  69,760  tons  were 
made  from  zincite  and  the  ores  associated  with  it.  This  had  an  esti- 
mated value  of  $9,626,991. 

THE  SESQUIOXIDES 

The  sesquioxides  (R2O3)  include  a  few  compounds  of  the  nonmetals 
that  are  comparatively  rare  and  a  group  of  metallic  compounds  that 
includes  two  minerals  of  great  economic  importance.  One  of  these, 
hematite  (Fe203),  is  the  most  valuable  of  the  iron  ores. 

ARSENOLITE— CLAUDETITE  GROUP 

The  only  group  of  the  nonmetallic  sesquioxides  that  need  be  referred 
to  in  this  place  comprises  those  of  arsenic  and  antimony.  This  is  an 
isodimorphous  group  including  four  minerals. 


Isometric 

Monoclinic 

Arsenolite 

AS2O3 

Claudetite 

Senarmontite 

Sb203 

Valentinite 

All  the  minerals  of  the  group  are  comparatively  rare.  The  isometric 
forms  occur  in.  well  developed  octahedrons  and  in  crusts  covering  other 
minerals.  They  are  also  found  in  earthy  masses.  It  is  probable  that  at 
high  temperatures  the  isometric  forms  pass  over  into  the  monoclinic 
modifications,  as  some  of  the  latter  have  been  observed  to  consist  of 
aggregates  of  tiny  octahedrons.     Crystals  of  claudetite  are  distinctly 


152  DESCRIPTIVE  MINERALOGY 

monoclinic,  but  they  are  so  twinned  as  to  possess  an  orthorhombic 
habit.  Valentinite  crystals,  on  the  contrary,  appear  to  be  plainly 
orthorhombic,  but  their  apparent  orthorhombic  symmetry  may  be 
due  to  submicroscopic  twinning  of  the  same  character  as  that  in 
claudetite,  but  which  in  the  latter  mineral  is  macroscopic. 

All  four  minerals  occur  as  weathered  products  of  compounds  contain- 
ing As  or  Sb.  They  give  the  usual  blowpipe  reactions  for  As  or  Sb. 
In  the  closed  tube  they  melt  and  sublime. 

Arsenolite  (AS2O3)  is  colorless  or  white.  Its  specific  gravity  is  3.7 
and  refractive  index  for  sodium  light  =  1.755.  It  usually  occurs  in  octa- 
hedrons; or  in  combinations  of  O(iu)  and  ooO(no),  but  these  when 
viewed  in  polarized  light  are  often  seen  to  be  anisotropic.  The  mineral  is 
found  also  in  aggregates  of  hair-like  crystals  with  a  hardness  of  1.2.  It  is 
soluble  in  hot  water,  yielding  a  solution  with  a  sweetish  taste. 

Senarmonite  (Sb203)  is  gray  or  white.  Its  density  is  5.2  and 
n=  2.087  f°r  yellow  light.  Its  octahedral  crystals  are  also  often  aniso- 
tropic; its  hardness  =2.  It  is  soluble  in  hot  HC1  but  is  only  very 
slightly  soluble  in  water.  When  heated  it  turns  yellow;  but  becomes 
white  again  upon  cooling. 

Claudetite  (AS2O3)  is  monoclinic  prismatic,  with  a  :  b  :  c  =  .4040  :  1 

:  .3445  and  0=86°  03'.     Its  white  crystals  are  usually  tabular  parallel 

to  00  P  00  (010)  and  are  twinned,  with  00  P  «>  (100)  the  twinning  plane. 

Their  cleavage  is  parallel  to   00 Pod  (010)  and  their  density  is  4.15. 

H=2.5.    The  mineral  is  an  electrical  nonconductor. 

Valentinite  (Sb20s)  is  apparently  orthorhombic  bipyramidal  (pos- 
sibly monoclinic  prismatic)  with  a  :  b  :  £=.3914  :  1  :  .3367.  Its  crystals 
are  tabular  or  columnar  in  habit  and  are  very  complex.  The  mineral  is 
found  also  in  radial  groups  of  acicular  crystals  and  in  granular  and 
dense  masses.  Its  color  is  white,  pink,  gray  or  brown,  and  streak 
white.  Its  density  is  5.77  and  hardness  2.5-3.  It  *s  insoluble  in  HC1. 
It  is  a  nonconductor  of  electricity. 

CORUNDUM   GROUP 

The  sesquioxides  of  aluminium  and  iron  constitute  an  isomorphous 
group  crystallizing  in  the  rhombohedral  division  of  the  hexagonal  sys- 
tem (ditrigonal  scalenohedral  class).  Both  the  aluminium  and  iron 
compounds,  corundum  and  hematite,  are  of  great  economic  importance. 


OXIDES  163 

Hematite  (Fe20a) 

Hematite  is  one  of  the  most  important  minerals,  if  not  the  most 
important  one,  from  the  economic  standpoint,  since  it  is  the  most  val- 
uable of  all  the  iron  ores.  It  is  known  by  its  dark  color  and  its  red 
powder.  It  occurs  in  black,  glistening  crystals,  in  yellow,  brown  or  red 
earthy  masses,  in  granular  and  micaceous  aggregates  and  in  botryoidal 
and  stalactitic  forms. 

Chemically,  the  mineral  is  Fe20s  corresponding  to  30  per  cent  O  and 
70  per  cent  Fe.  In  addition  to  these  constituents,  hematite  often  con- 
tains some  magnesium  and  some  titanium.  By  increase  in  the  latter 
element  it  passes  into  a  mineral  which  has  not  been  distinguished  from 
ilmenite  (see  p.  462). 

The  habit  of  hematite  crystals  is  nearly  always  rhombohedral. 


Fig.  6$. — Hematite  Crystals  with  R,  10T1  (r);  |p2f  2243  (*);  JR  10T4  (u);  00  P2, 

1 1 20  (a)  and  oR,  0001  (c). 

Their  axial  ratio  is  a:  c=i  :  1.3658,  and  the  predominant  forms  are 
R(ioTi),  iR(iol4),  |P2(2243),  the  prisms  ooP(ioTo)and  <x>P2(ii2o) 
and  often  the  basal  plane  (Fig.  65).  In  addition,  about  no  other  forms 
have  been  identified.  The  crystals  are  often  tabular,  and  sometimes 
are  grouped  into  aggregates  resembling  rosettes.  In  many  cases  the 
terminal  faces  are  rounded.  A  parting  is  often  observed  parallel  to 
the  basal  plane,  due  to  the  occunence  of  the  mineral  in  aggregates  in 
which  each  crystal  is  tabular. 

Hematite  has  no  well  defined  cleavage.  Its  fracture  is  conchoidal  or 
earthy.  Its  crystals  are  black,  glistening  and  opaque,  except  in  very 
small  splinters.  These  are  red  and  transparent  or  translucent.  Earthy 
varieties  are  red.  The  streak  of  all  varieties  is  brownish  red  or  cherry- 
red.  The  hardness  of  the  crystallized  hematite  is  5.5-6.5  and  its  density 
about  5.2.  It  is  a  good  conductor  of  electricity.  Its  refractive  indices 
are:  (0=3.22,  €=2.94  for  yellow  light. 

The  mineral  is  infusible  before  the  blowpipe.  In  the  reducing  flame 
on  charcoal  it  becomes  magnetic,  and  when  heated  with  soda  it  is  reduced 
to  a  magnetic  metallic  powder.    It  is  soluble  in  strong  hydrochloric  acid. 


154  DESCRIPTIVE  MINERALOGY 

The  crystalline  and  earthy  aggregates  of  hematite  to  which  distinct 
names  have  been  given  are: 

Specular,  when  the  aggregate  consists  of  grains  with  a  glistening, 
metallic  luster,  like  the  luster  of  the  crystals.  When  the  grains  are  thin 
tabular  the  aggregate  is  said  to  be  micaceous. 

Columnar  or  fibrous,  when  in  fibrous  masses.  The  color  is  usually 
brownish  red  and  the  luster  dull.  The  botryoidal,  stalactic  and  various 
imitative  forms  belong  here.  Red  hematite  is  a  compact  red  variety  in 
which  the  fibrous  structure  is  not  very  pronounced. 

Red  ocher  is  a  red  earthy  hematite  mixed  with  more  or  less  clay  and 
other  impurities. 

Clay  ironstone  is  a  hard  brownish  or  reddish  variety  with  a  dull  luster. 
It  is  usually  a  mixture  of  hematite  with  sand  or  clay. 

Oolitic  ore  is  a  red  variety  composed  of  compacted  spherical  or  nearly 
spherical  grains  that  have  a  concentric  structure. 

Fossil  ore  differs  from  oolitic  ere  mainly  in  the  fact  that  there  are 
present  in  it  small  shells  and  fragments  of  shells  that  are  now  composed 
entirely  of  hematite. 

Mar  tile  is  a  pseudomorph  of  hematite  after  magnetite. 

Hematite  is  distinguished  from  all  other  minerals  by  its  red  powder 
and  its  magnetism  after  roasting. 

Syntheses. — Crystals  of  hematite  are  obtained  by  the  action  of  steam 
on  ferric  chloride  at  red  heat;  by  heating  ferric  hydroxide  with  water 
containing  a  trace  of  NH4F  to  2500  in  a  closed  tube,  and  by  cooling  a 
solution  of  Fe2<33  in  molten  borax  or  halite. 

Occurrence  and  Origin. — Hematite  is  found  in  beds  with  rocks  of 
nearly  all  ages.  It  occurs  also  as  a  deposit  on  the  bottoms  of  marshy 
ponds,  and  in  small  grains  in  the  rocks  around  volcanic  vents.  The 
crystallized  variety  is  often  deposited  on  the  sides  of  clefts  in  rocks  near 
volcanoes  and  on  the  sides  of  certain  veins.  It  is  produced  by  sublima- 
tion, by  sedimentation  and  by  metasomatic  processes. 

Localities. — Handsome  crystals  occur  on  the  island  of  Elba;  near 
Limoges  in  France;  in  and  on  the  lavas  of  Vesuvius  and  Etna;  at  many 
places  in  Switzerland,  Sweden,  etc.,  and  at  many  in  the  United  States. 

Beds  of  great  economic  importance  occur  in  the  Gogebic,  Menominee 
and  Marquette  districts  in  Michigan;  in  the  Mesabe  and  Vermilion 
districts  in  Minnesota;  in  the  Pilot  Knob  and  Iron  Mountain  districts 
in  Missouri,  and  in  the  southern  Appalachians,  especially  in  Alabama. 

Uses. — In  addition  to  its  use  as  an  ore  the  fibrous  variety  of  hematite 
is  sometimes  cut  into  balls  and  cubes  to  be  worn  as  jewelry.  The  earthy 
varieties  are  ground  and  employed  in  the  manufacture  of  a  dark  red 


OXIDES  155 

paint  such  as  is  used  on  freight  cars,  and  the  powder  of  some  of  the  mass- 
ive forms  is  used  as  a  polishing  powder. 

Production. — Most  of  the  iron  ore  produced  in  the  United  States  is 
hematite,  and  by  far  the  greater  proportion  of  it  comes  from  the  Lake 
Superior  region.    The  statistics  for  191 2  follow: 

Quantity  (in  Long  Tons)  of  Iron  Ore  Mined  in  the  Several  Lead- 
ing States  During  1912 

Hematite             Other  Iron  Ores  Total 

Minnesota 34,431,000  34431,000 

Michigan 11,191,000  11,191,000 

Alabama 3,814,000                   749,ooo  4,563,000 

New  York 106,327  1,110,000  1,216,327 

Wisconsin 860,000  860,000 

Tennessee 246,000                    171,000  417,000 

Total  in  U.  S 51,345,782  3,804,365  55,i5o,i47 

The  total  production  in  191 2  was  valued  at  about  $104,000,000. 

Corundum  (AI2O3) 

Corundum  is  the  hardest  mineral  known,  with  the  exception  of  dia- 
mond. In  consequence  of  its  great  hardness  an  impure  variety  is  used 
as  an  abrading  agent  under  the  name  of  emery.  It  is  also  one  of  the 
most  valuable  of  the  gem  minerals.  It  occurs  as  crystals  and  in  granular 
masses. 

The  mineral  is  nearly  always  a  practically  pure  oxide  of  aluminium  of 
the  composition  AI2O3,  in  which  there  are  52.9  per  cent  Al  and  47.1  per 
cent  O.  The  impure  varieties  usually  contain  some  iron,  mainly  as  an 
admixture  in  the  form  of  magnetite. 

The  axial  ratio  of  corundum  crystals  is  1  :  1.36.  The  forms  are 
usually  simple  pyramids,  among  which  ^2(2243)  an^  1^2(4483) 
are  the  most  common  (Fig.  66),  and  the  prism  00  P2(ii2o).  The  basal 
plane  is  also  common  (Fig.  67).  Many  crystals  consist  of  a  series  of 
steep  prisms  and  the  basal  plane,  with  a  habit  that  may  be  described  as 
barrel-shaped  (Fig.  68).  The  crystals  are  often  rough  with  rounded 
edges.  The  prismatic  and  pyramidal  faces  are  usually  striated  hori- 
zontally, and  the  basal  plane  by  lines  radiating  from  the  center. 

All  corundum  crystals  are  characterized  by  a  parting  parallel  to  the 
basal  plane,  and  often  by  a  cleavage  parallel  to  the  rhombohedron,  due 
to  the  presence  of  lamellae  twinned  parallel  to  R(ioTi).  The  fracture 
of  the  mineral  is  conchoidal  or  uneven.    Its  density  is  about  4  and  its 


156 


DESCRIPTIVE  MINERALOGY 


hardness  9.  The  mineral  possesses  a  vitreous  to  adamantine  luster.  It 
is  transparent  or  translucent.  Its  streak  is  uncolored.  Its  color  varies 
from  white,  through  gray  to  various  shades  of  red,  yellow,  or  blue. 
The  blue  varieties  are  pleochroic  in  blue  and  greenish  blue  shades.  The 
mineral  is  a  nonconductor  of  electricity.  Its  refractive  indices  for 
yellow  light  are:  «=  1.7690,  *=  1.7598. 

Three  varieties  of  corundum  are  recognized  in  the  arts:  Sapphire, 
corundum  and  emery. 

Sapphire  is  the  generic  name  for  the  finely  colored,  transparent  or 
translucent  varieties  that  are  used  as  gems,  watch  jewels,  meter  bearings, 
etc.    The  sapphires  are  divided  by  the  jewelers  into  sapphires,  possessing 


Fig.  66. 


Fig.  67. 


Fig.  68. 


Fig.  66. — Corundum  Crystal  with  jP2,  4483  (v). 

Fig.  67. — Corundum  Crystal  with  R,  10T1  (r);    00  P2,  1120  (a),  and  oR,  0001  (c). 

Fig.  68.— Corundum  Crystal.     Form  a,  v  and  c  as  in  previous  figures.    Also  {P2, 

2243  (»)  and  —  2R,  0221  (j). 


a  blue  color,  rubies,  possessing  a  red  shade,  Oriental  topazes,  Oriental 
emeralds  and  Oriental  amethysts  having  respectively  yellow,  green  and 
purple  tints. 

Corundum  is  the  name  given  to  dull  colored  varieties  that  are  ground 
and  used  as  polishing  and  cutting  materials. 

Emery  is  an  impure  granular  corundum,  or  a  mixture  of  corundum 
with  magnetite  (Fe304)  and  other  dark  colored  minerals.  Emery,  like 
corundum,  is  used,  as  an  abrasive.  It  is  less  valuable  than  corundum 
powder  because  it  contains  a  large  proportion  of  comparatively  soft 
material. 

Powdered  corundum  when  heated  for  a  long  time  with  a  few  drops  of 
cobalt  nitrate  solution  assumes  a  blue  color.  The  mineral  gives  no 
definite  reaction  with  the  beads      It  is  infusible  and  insoluble.     It  is 


OXIDES  157 

most  easily  recognized  by  its  hardness.  The  mineral  alters  to  spinel 
(p.  196)  and  to  fibrous  and  platy  aluminous  silicates. 

Syntheses. — Corundum  crystals  have  been  produced  artificially  in 
many  different  ways,  but  only  recently  has  the  manufacture  of  the  gem 
variety  been  accomplished  on  a  commercial  scale.  Amorphous  AI2C3 
dissolves  in  melted  sodium  sulphide  and  crystallizes  from  the  glowing 
mass  at  a  red  heat.  By  melting  AI2Q3  in  a  mass  of  some  fluoride  and 
potassium  carbonate  containing  a  little  chromium,  and  using  compara- 
tively large  quantities  of  material,  violet  and  blue  rubies  were  obtained 
by  Fremy  and  Verneuil.  Rubies  are  also  produced  by  melting  AI2O3 
and  a  little  Cr203  for  several  minutes  at  a  temperature  of  22500  C.  in 
an  electric  oven. 

In  recent  years  reconstructed  rubies  have  become  a  recognized  article 
of  commerce.  These  are  crystalline  drops  of  ruby  material  made  by 
melting  tiny  splinters  and  crystals  of  the  mineral  in  an  electric  arc. 

Alundum  is  an  artificial  corundum  made  by  subjecting  the  aluminium 
hydroxide,  bauxite,  to  an  intense  heat  (5ooo°-6ooo°)  in  an  electric 
furnace. 

Occurrence  and  Origin. — Corundum  usually  occupies  veins  in  crys- 
talline rocks  or  is  embedded  in  basic  intrusive  rocks  and  in  granular 
limestone.  The  sapphire  varieties  are  also  often  found  as  partially 
rounded  crystals  in  the  sands  of  brook  beds.  The  varieties  found  in 
igneous  rocks  are  primary  crystallizations  from  the  magmas  producing 
the  rocks.  The  varieties  in  limestones  are  the  result  of  metamorphic 
processes. 

Localities. — Sapphires  are  obtained  mainly  from  the  limestone  of 
Upper  Burma.  They  are  known  also  to  occur  in  Afghanistan,  in  Kash- 
mir and  in  Ceylon.  They  are  occasionally  found  in  the  diamond-bearing 
gravels  of  New  South  Wales  and  in  the  bed  of  the  Missouri  River,  near 
Helena,  Montana.  In  the  United  States  sapphire  is  mined  near  the 
Judith  River  in  Fergus  Co.,  and  in  Rock  Creek  in  Granite  Co.,  Mont., 
where  it  occurs  in  a  dike  of  the  dark  igneous  rock  known  as  monchiquite, 
and  is  washed  from  the  placers  of  three  streams  in  the  same  State.  The 
only  southern  mines  that  have  produced  gem  material  are  at  Franklin 
and  Culsagee,  N.  C,  and  from  these  not  any  great  quantity  of  stones  of 
gem  quality  have  been  taken. 

The  largest  sapphire  crystal  ever  found  was  taken,  however,  from 
one  of  them.  It  weighs  312  lb.,  is  blue,  but  opaque.  From  one  of 
these  mines,  also,  came  the  finest  specimen  cf  green  sapphire  (Oriental 
emerald)  ever  found. 

Corundum  in  commercial  quantities  occurs  on  the  coast  of  Malabar, 


158  DESCRIPTIVE  MINERALOGY 

in  Siam,  near  Canton,  China,  and  in  southeastern  Ontario,  Canada. 
Emery  is  obtained  from  several  of  the  Grecian  Islands,  more  particularly 
Naxos,  and  from  Asia  Minor.  It  is  mined  in  the  United  States  at  Chester, 
Mass.,  and  at  Peekskill,  N.  Y.  Crystallized  corundum  occurs  near 
Litchfield,  Conn.;  at  Greenwood,  Maine;  at  Warwick  and  Amity,  N.  Y. ; 
at  Mineral  Hill,  Penn.;  in  Patrick  Co.,  Va.;  at  Corundum  Hill  and  at 
Laurel  Creek,  Macon  Co.,  N.  C,  and  at  various  points  in  Georgia,  at 
ail  of  which  places  it  has  been  mined.  In  all  the  localities  within  the 
United  States  the  corundum  occurs  on  the  peripheries  of  masses  of 
peridotite  (olivine  rocks). 

Uses. — Corundum,  emery  and  alundum,  after  crushing  and  washing, 
are  used  as  abrasives  and  in  the  manufacture  of  cutting  wheels. 

Production. — The  amount  of  sapphire  produced  in  the  United  States 
in  191 2  was  valued  at  $195,505.  Most  of  it  was  used  for  mechanical 
purposes,  but  384,000  carats  were  used  as  gem  material. 

Most  of  the  corundum  used  in  the  United  States  is  imported  from 
Canada,  where  it  occurs  in  Haliburton,  Renfrew  and  neighboring  coun- 
ties in  Ontario,  as  crystals  scattered  through  the  coarse-grained  crys- 
talline rocks  known  as  syenite,  nepheline  syenite  and  anorthosite. 

Most  of  the  emery  is  also  imported.  Only  992  tons  with  a  value 
of  $6,652  were  mined  in  191 2.  The  imports  of  corundum  and  emery 
were  valued  at  $501,725,  but  the  importation  of  these  substances  is 
gradually  diminishing  because  of  the  rapid  increase  in  the  amounts 
of  alundum  and  carborundum  manufactured.  In  191 2  the  production 
of  alundum  reached  13,300.000  lb.  valued  at  $796,000, 

THE  DIOXIDES 

THE  NONMETALLIC  DIOXIDES 

There  are  but  few  dioxides  of  the  nonmetals  that  occur  as  minerals, 
and  only  one  of  these,  quartz,  is  abundant. 

SILICA  GROUP 

Silica  (Si02)  occurs  in  nature  in  four  or  five  important  modifica- 
tions as  follows: 

a  Quartz,  trigonal-trapezohedral  class,  below  5750. 

j8  Quartz,  hexagonal-trapezohedral  class,  above  575 °  and  below  8700. 

Tridymite,  rhombic  bipyramidal,  pseudohexagonal  habit.  Hex- 
agonal above  1170. 

Cristobalite,  tetragonal  system,  pseudocubic  habit.  Isometric  above 
1400. 


OXIDES 


159 


Chalcedony  is  regarded  by  many  mineralogists  as  a  form  of  quartz, 
but  its  index  of  refraction  for  red  light  is  »=  1.537,  which  is  noticeably 
lower  than  that  of  either  ray  in  quartz,  which  is  u= 1.5390,  €=1.5480 
for  the  same  color.  Its  hardness  also  is  a  little  less  than  that  of  quartz. 
Some  mineralogists  believe  that  all  of  these  properties  may  be  explained 
on  the  assumption  that  the  mineral  is  a  mass  of  fine  quartz  fibers,  perhaps 
mixed  with  other  substances,  but  those  who  have  investigated  it  by 
high  temperature  methods  are  inclined  to  regard  it  as  a  distinct  mineral. 

Quartz  (Si02) 

Quartz  vies  with  calcite  for  the  commanding  position  among  the 
minerals.    It  is  very  abundant,  and  appears  under  a  great  variety  of 


A 


B 


Fig.  69. 


Fig.  70. 


Fig.  69.— Quartz  Crystal  Exhibiting  Rhombohedral  Symmetry.    R,  ion  (r);  — R, 

01T1  (2)  and  °°R,  10T0  (m).  "» 

Fig.  70. — Ideal  (A)  and  Distorted  (B)  Quartz  Crystals  Bounded  by  same  Forms  as 

in  Fig.  69. 


forms.  Often  it  occurs  in  distinct  crystals.  At  other  times  it  appears 
as  grains  without  distinct  crystal  forms,  and  again  it  constitutes  great 
massive  deposits. 

Pure  quartz  consists  of  46.7  per  cent  Si  and  53.3  per  cent  O.  Mass- 
ive varieties  often  contain,  in  addition,  some  opal  (Si(OH)4),  and  traces 
of  iron,  calcite  (CaCOs),  clay,  and  other  impurities. 

The  crystallization  of  quartz  is  in  the  trapezohedral  tetartohedral 
division  of  the  hexagonal  system  (trigonal-trapezohedral  class),  at  tem- 
peratures below  5750.  When  formed  above  this  temperature  its  sym- 
metry is  hexagonal  trapezohedral  (hemihedral).  The  former  is  known  as 
a  quartz,  and  the  latter  as  0  quartz.  They  readily  pass  one  into  the 
other  at  the  stated  temperature.    The  axial  ratio  is  1  :  i.x.    The  prin- 

-                 -                  -      2P2      _ 
cipal  forms  observed  are  +R(ioii),  — R(oni),  00  R(ioio), (1121), 


160  DESCRIPTIVE  MINERALOGY 

6P( 

— -=<5i5i)  (Fig-  74)  and  a  series  of  steep  rhombohedrons  and  trapezo- 

hedrons.     Although  these  may  all  be  tetartohedral  since  ibe  geometrical 


Fig.  71. — Etch  Figures  on  Two  Quartz  Crystals  of  the  Same  Form,  Illustrating  Dif- 
ferences in  Symmetry.  A.  Right- Hand  Crystal.  B.  Left -Hand  Crystal. 
(Afltr  Ptnfidd.) 


Fie.  72. — Group  of  Quartz  Crystals  with  Distorted  Rhombohedral  Faces.     {Foote 
Mineral  Company.) 

forms  of  the  first  four  are  not  distinguishable  from  the  corresponding 
hemihedral  ones,  the  crystals  possess  a  rhombohedral  symmetry  (Fig. 
69).    The  angle  10T1  a  ^toi  =  85°  46'. 


OXIDES 


161 


.  Often  the  +R  and  the  — R  faces  are  equally  developed  so  that  they 
appear  to  belong  to  the  hexagonal  pyramid  P  (Fig.  70A).  Their  true 
character,  however,  is  clearly  brought  out  by  etching,  when  figures  are 
produced  on  the  +R  and  the  — R  that  are  differently  situated  with 
respect  to  the  edges  of  the  faces  (Fig.  71).  On  the  other  hand,  on  many 
crystals  some  of  the  R  faces  are  very  much  enlarged  at  the  expense  of 
the  others  (Fig.  7a). 

The  crystals  are  commonly  prismatic.     Often   they   are  so  dis- 


V 


Fig.  73. 


Fig.  ; 


Fig.  73. — Taperini;  Quartz  Crystal  with  Rhombohcdral  Symmetry.     A   Combination 

of  f ,  e,  m  and  Two  Steep  Rhombohedrons.     B.  Cross-section  near  Top. 
Fig.  74.— Quartz  Crystals  Containing  °oR,  10T0  (m);   R,  10T1  (r);  -R,  01T1  (i); 


on  B. 


1  (f); 


torted  that  it  is  difficult  to  detect  the  position  of  the  c  axis  (Fig. 
70B).  The  striations  on  00  R{ioTo)  are,  however,  always  parallel  to 
the  edges  between  R  and  00  R.  When  these  are  sharply  marked  the 
position  of  the  vertical  axis  is  easily  recognized.  Many  crystals 
taper  sharply  toward  the  ends  of  the  c  axis.  This  tapering  is  due  to 
oscillatory  combination  of  the  prism  00  R  with  rhombohedrons 
(Fig.  73). 

The  habits  of  the  crystals  vary  with  the  crystallization  of  the  quartz. 
On  crystals  of  the  0  phase  the  +R  and  —  R  faces  are  equally  developed 
and  trigonal  trapezohedrons  are  absent.    The  crystals  are  hexagonal  in 


162 


DESCRIPTIVE  MINERALOGY 


habit.    Crystals  of  the  a  phase  usually  exhibit  marked  differences  in 

the  size  and  character  of  the  rhombohedral  planes;  and  trigonal  trape- 

zohedrons  may  be  present  on  them.    Such  crystals  are  usually  trigonal 

in  habit  and  prismatic. 

2P2      _ 

The  small (n  21)  faces  on  all  types  of  crystals  (Fig.  74)  are 

2 

always  striated  parallel  to  the  edge  between  this  plane  and  +R.    By 

their  aid  the  +R  can  always  be  distinguished  from  the  —  R.    This  is  a 

matter  of  some  practical  importance  since  plates  cut  from  quartz  crystals 

possess  the  power  of  rotating  a  ray  of  polarized  light.    The  plates  cut 


Fig.  75. — Supplementary  Twins  of  Quartz. 

C  is  a  combination  of  A  and  B  in  Fig.  74  twinned  about  00  P2(ii2o).  This  is 
known  as  the  Brazil  law. 

D  is  a  combination  of  two  crystals  like  B  twinned  about  c  as  the  twinning  axis. 
One  is  revolved  6o°  with  reference  to  the  others,  thus  causing  the  r  and  z  faces  to 
fall  together.  Swiss  law.  E  is  a  twin  like  D,  showing  portions  of  planes  belonging 
to  each  individual.     It  contains  also  the  form  s. 


from  some  crystals  turn  the  ray  to  the  right;  those  cut  from  others  turn 

it  to  the  left.     Crystals  that  produce  plates  of  the  first  kind  are  known 

as  right-handed  crystals;  those  that  produce  plates  of  the  second  kind  as 

left-handed  crystals.     Since  this  property  of  quartz  plates  is  employed 

in  the  construction  of  optical  instruments  for  use  in  the  detection  of 

sugars  and  certain  other  substances  in  solution  it  is  important  to  be 

able  to  distinguish  those  crystals  that  will  yield  right-handed  plates  from 

those  that  will  yield  left-handed  ones.    Observation  has  shown  that 

2P2      _ 

when  the (n  21)  faces  are  in  the  upper  right-hand  corner  of  the  00  R 

2 

plane  immediately  beneath  +R  the  crystal  is  right-handed.     When 

these  faces  are  in  the  upper  left-hand  corner  of  this  00  R  plane  the  crystal 


I nterpenet ration  twins  of  quartz  are  so  common  that  few  crystals 
can  be  observed  that  do  not  exhibit  some  evidence  of  twinning  (Fig.  75). 
The  twinning  plane  is  «  R,  so  that  the  c  axes  in  the  twinned  individuals 
are  parallel  and,  indeed,  often  coincident.  The  R  faces  and  the  00  R 
faces  practically  coincide  in  the  twinned  parts  so  that  the  crystals 
resemble  untwinned  ones.  The  twinning  is  exhibited  by  dull  areas  of 
—  R  on  bright  areas  of  +R  faces  and  by  breaks  in  the  continuity  of  the 
striations  on  00  R. 

Other  twinning  laws  have  also  been  observed  in  quartz,  but  their 
discussion  as  well  as  the  more  complete  discussion  of  the  mineral's 
crystallization  must  be  left  for  larger  treatises.     In  the  most  common  of 
these  other  laws  the  individuals  are  twinned  about 
Pa(n«).     See  Fig.  76. 

The  fracture  of  quartz  is  conchoidal.  Its  hard- 
ness is  7  and  density  2.65.  Its  luster  is  vitreous,  or 
sometimes  greasy.  Pure  specimens  are  transparent 
or  colorless,  but  most  varieties  are  colored  by  the 
addition  of  pigments  or  impurities.  When  the 
coloring   matter  is  opaque  it   may  be   present    in 

sufficient  quantity  to  render  the  mineral  also  opaque.  _  ,     _ 

7.        ,    ,       .  .    .  ,    ,  Fie.    ;6.— Quart). 

The  streak  is  colorless  in  pure  varieties,  and  of  some     jwinne^   about 
pale  shade  in  colored  varieties.    The  mineral  is  pyro-     Pi(u5j). 
electric  and  circularly  polarizing  as  described  above. 
It  is  an   electric  insulator  at   ordinary  temperatures.     Its  refractive 
indices  for  yellow  light  are:  w=  1.5443.  *=  1-5534- 

Quartz  resists  most  of  the  chemical  agents  except  the  alkalies.  It 
dissolves  in  fused  sodium  carbonate  and  in  solutions  of  the  caustic 
alkalies.  It  is  also  soluble  in  HF  and  to  a  very  slight  degree  in  water, 
especially  in  water  containing  small  quantities  of  certain  salts.  When 
heated  to  575°  the  a  variety  passes  into  the  0  variety;  at  8700  both 
varieties  pass  into  tridymite,  and  at  14700  the  tridymite  passes  over  into 
cristobalite.     Gradual  fusion  occurs  just  below  1470°. 

The  varieties  of  quartz  have  received  many  different  names  depend- 
ing largely  upon  their  color  and  the  uses  to  which  they  are  put.  They 
may  be  grouped  for  convenience  into  crystallized  and  crystalline  vari- 
eties. 

The  principal  crystallized  varieties  are: 


164  DESCRIPTIVE  MINERALOGY 

Rock  crystal,  the  colorless,  transparent  variety,  that  often  forms 
distinct  crystals.  This  is  the  variety  that  is  used  in  optical  instruments. 
It  includes  the  Lake  George  diamonds,  rhinestones  and  Brazilian  peb- 
bles. 

Amethyst,  the  violet-colored  transparent  variety. 

Rose  quartz,  the  rose-colored  transparent  variety. 

Citrine  ox  false  topaz,  a  yellow  and  pellucid  kind. 

Smoky  quartz  or  Cairngorm  stone,  a  smoky  yellow  or  smoky  brown 
variety  that  is  often  transparent  or  translucent,  but  sometimes  almost 
opaque. 

The  last  four  varieties  are  used  as  gems,  the  Cairngorm  stone  being  a 
popular  stone  for  mourning  jewelry. 

Milky  quartz  is  the  white,  translucent  or  opaque  variety  such  as  so 
commonly  forms  the  gangue  in  mineral  veins  and  the  material  of  "  quartz 
veins." 

Sagenite  is  rock  crystal  including  acicular  crystals  of  rutile  (Ti02). 

Aventurine  is  rock  crystal  spangled  with  scales  of  some  micaceous 
mineral. 

The  principal  crystalline  varieties  are: 

Chalcedony,  a  very  finely  fibrous,  transparent  or  translucent  waxy- 
looking  quartz  that  forms  mamillary  or  botryoidal  masses.  Its  color  is 
white,  gray,  blue  or  some  other  delicate  shade.  The  water  that  is  always 
present  in  it  is  believed  to  be  held  between  the  minute  fibers,  and  not  to 
be  combined  with  the  silica  (see  also  p.  159). 

Carnelian  is  the  name  given  to  a  clear  red  or  brown  chalcedony. 

Chrysoprase  is  an  apple-green  chalcedony. 

Prase  is  a  dull  leek- green  variety  that  is  translucent. 

Plasma  differs  from  prase  in  having  a  brighter  green  color  and  in 
being  translucent. 

Heliotrope,  or  bloodstone,  is  a  plasma  dotted  with  red  spots  of  jasper. 

All  of  the  colored  chalcedonies  are  used  as  gems  or  as  ornamental 
stones. 

Agate  is  a  chalcedony,  or  a  mixture  of  quartz  and  chalcedony,  varie- 
gated in  color.  The  commonest  agates  have  the  colors  arranged  in 
bands,  but  there  are  others,  like  "  fortification  agate  "  in  which  the 
colors  are  irregularly  distributed,  and  still  others  in  which  the  variation 
in  color  is  due  to  visible  inclusions,  as  in  "  moss-agates.' '  The  different 
bands  in  banded  agates  often  differ  in  porosity.  This  property  is  taken 
advantage  of  to  intensify  the  contrast  in  their  colors.  The  agate  is 
soaked  in  oil,  or  in  some  other  substance,  and  is  then  treated  with  chem- 
icals that  act  upon  the  material  absorbed  by  it.    Those  bands  which 


OXIDES  165 

have  absorbed  the  greater  quantity  of  this  material  become  darker  in 
color  than  those  that  have  absorbed  less. 

Onyx  is  a  very  evenly  banded  agate  in  which  there  is  a  marked  con- 
trast in  colors.  Cameos  are  onyxes  in  one  band  of  which  figures  are  cut, 
leaving  another  band  to  form  a  background. 

Sardonyx  is  an  onyx  in  which  some  of  the  bands  consist  of  carnelian. 
It  is  usually  red  and  white. 

Flint,  jasper,  hornstone  and  touchstone  are  very  fine  grained  crystalline 
aggregates  of  gray,  red  or  nearly  black  mixture  of  opal,  chalcedony  and 
quartz.  They  are  more  properly  rocks  than  minerals.  Chert  is  an  im- 
pure flint. 

Sandstone  is  a  rock  composed  of  sand  grains,  most  of  which  are 
quartz,  cemented  by  clay,  calcite  or  some  other  substance.  When  the 
cement  is  quartz  the  rock  is  a  quartzite.  Oilstones,  honestones  and  some 
whetstones  are  cryptocrystalline  aggregates  of  quartz,  very  dense  and 
homogeneous,  except  for  tiny  rhombohedral  cavities  that  are  thought  to 
have  resulted  from  the  solution  of  crystals  of  calcite.  They  are  gener- 
ally believed  to  be  beds  of  metamorphosed  chert. 

Syntheses.— Crystallized  quartz  has  been  made  in  a  number  of  ways 
both  from  superheated  aqueous  solutions  and  from  molten  magmas. 
Crystals  have  been  produced  by  the  action  of  water  containing  am- 
monium fluoride  upon  powdered  glass  and  upon  amorphous  Si02,  and 
by  heating  water  in  a  closed  glass  tube  to  high  temperatures.  The 
separation  of  crystals  from  molten  magmas  is  facilitated  by  the  addition 
of  small  quantities  of  a  fluoride  or  of  tungsten  compounds. 

Occurrence  and  Origin. — Quartz  occurs  as  an  essential  constituent  of 
many  crystalline  rocks  such  as  granite,  gneiss,  etc.,  and  as  the  almost  sole 
component  of  certain  sandstones.  It  constitutes  the  greater  portion  of 
most  sands  and  the  material  of  many  veins.  It  also  occurs  as  pseudo- 
morphs  after  shells  and  other  organic  bodies  embedded  in  rocks,  having 
replaced  the  original  substance  of  which  these  bodies  were  composed. 
It  is  also  one  of  the  decomposition  products  of  many  silicates.  It  may 
thus  be  primary  or  secondary  in  origin.  It  may  result  from  igneous  or 
aqueous  processes,  or  it  may  be  a  sublimation  product. 

Localities. — Quartz  is  so  widely  spread  in  its  distribution  that  only  a 
very  few  of  its  most  interesting  localities  can  be  referred  to  in  this  place. 

The  finest  specimens  of  rock  crystals  come  from  Dauphin6,  France; 
Carrara,  in  Tuscany;  the  Piedmont  district,  in  Italy;  and  in  the  United 
States  from  Middleville,  and  Little  Falls,  N.  Y.;  the  Hot  Springs, 
Ark.,  and  from  several  places  in  Alexander  Co.,  N.  C.  Smoky  quartz 
is  found  in  good  crystals  in  Scotland;  at  Paris,  Me.;  in  Alexander 


166  DESCRIPTIVE  MINERALOGY 

♦ 

Co.,  N.  C,  and  in  the  Pike's  Peak  region  of  Colorado.  The  handsomest 
amethysts  come  from  Ceylon,  Persia,  Brazil,  Nova  Scotia  and  the 
country  around  Lake  Superior.  Rose  quartz  occurs  in  large  quantity 
at  Hebron,  Paris,  Albany  and  Georgetown,  Me. 

Fine  agates  and  carnelians  are  brought  from  Arabia,  India  and  Brazil. 
They  are  abundant  in  the  gravels  of  Agate  Bay  and  of  other  bays  and 
coves  on  the  north  shore  of  Lake  Superior. 

Chalcedony  is  abundant  in  the  rocks  of  Iceland  and  the  Faroe  Islands, 
in  those  on  the  northwest  side  of  Lake  Superior,  and  in  the  gravels  of 
the  Columbia,  the  Mississippi  and  other  western  rivers. 

The  other  valuable  varieties  of  the  mineral  occur  largely  in  the  Far 
East. 

Agatized,  or  silicified,  wood  of  great  beauty  exists  in  enormous  quan- 
tity in  an  old  petrified  forest  near  Corrizo,  Aiiz.  It  is  also  found  in 
the  Yellowstone  Park;  near  Florissant,  Colo.,  and  in  other  places  in  the 
Far  West.  This  wood  has  had  all  of  its  organic  matter  replaced  mole- 
cule for  molecule  by  quartz  in  such  a  manner  that  its  original  structure 
has  been  perfectly  preserved. 

Uses. — Rock  crystal  is  used  more  or  less  extensively  in  the  construc- 
tion of  optical  instruments  and  in  the  manufacture  of  cheap  jewelry. 
Smoky  quartz,  amethyst,  onyx,  carnelian  and  heliotrope  stones  are 
used  as  gems,  and  agate,  prase,  chrysoprase  and  rose  quartz  as  orna- 
mental stones. 

Milky  quartz,  ground  to  coarse  powder,  is  employed  in  the  manu- 
facture of  sandpaper.  Its  most  extensive  use,  however,  is  in  the  man- 
ufacture of  glass  and  pottery.  Earthenware,  porcelain  and  some  other 
varieties  of  potter's  ware  are  vitrified  mixtures  of  clay  and  ground 
quartz,  technically  known  as  "flint."  Ordinary  glass  is  a  silicate  of 
calcium  or  lead  and  the  alkalies,  sodium  or  potash.  It  is  made  by 
melting  together  soda,  potash,  lime  or  lead  oxide  and  ground  quartz  or 
quartz  sand,  and  coloring  with  some  metallic  salt.  A  pure  quartz  glass 
is  now  being  made  for  chemical  uses  by  melting  pure  quartz  sand. 

Quartz  is  sometimes  used  as  a  flux  in  smelting  operations.  In  the 
form  of  sandstone,  it  is  used  as  a  building  stone,  and  in  the  form  of  sand 
it  is  employed  in  various  building  operations.  Bricks  cut  from  dense 
quartzites  (very  hard  and  compact  sandstones)  are  often  employed 
for  lining  furnaces. 

The  uses  of  honestones,  oilstones,  and  whetstones  are  indicated  by 
their  names. 

Production. — Many  varieties  of  quartz  are  produced  in  the  United 
States  to  serve  various  uses.    Vein  quartz  is  crushed  and  employed 


OXIDES  167 

in  the  manufacture  of  wood  filler,  paints,  pottery,  scouring  soaps,  sand- 
paper and  abrasives.  It  is  also  used  in  making  ferro-silicon,  chemical 
ware,  pottery,  sand-lime  brick,  quartz  glass,  etc.  The  total  quantity 
produced  for  these  purposes  in  191 2  was  97,874  tons,  valued  at 
$191,685. 

The  largest  quantity  of  quartz  produced  is  in  the  form  of  sand,  of 
which  38,600,000  tons  were  marketed  in  1912  at  a  valuation  of  $15,300,- 
000.  Sandstone,  valued  at  $6,900,000,  was  quarried  for  building  and 
paving  purposes.  Oilstones,  grindstones,  millstones,  etc.,  which  are 
made  from  special  varieties  of  sandstone,  were  produced  to  the  value  of 
$1,220,000. 

Gem  quartz  obtained  in  191 2  was  valued  at  about  $22,000.  This 
comprised  petrified  wood,  chrysoprase,  agate,  amethyst,  rock  crystal, 
smoky  quartz,  rose  quartz,  and  gold  quartz  (white  quartz  containing 
particles  of  gold). 

THE  METALLIC  DIOXIDES 

The  metallic  dioxides  include  the  oxides  of  tin,  titanium,  manganese 
and  lead.  Of  these  the  manganese  dioxide  may  be  dimorphous,  and  the 
titanium  dioxide  is  trimorphous.  A  dioxide  of  zirconium  is  also  known, 
badddeyite,  but  it  is  extremely  rare.  The  mineral  zircon  (ZrSiOO  is 
often  regarded  as  being  isomorphous  with  cassUerite  (Sn02)  and  rutile 
(Ti02)  because  of  the  similarity  in  the  crystallization  of  the  three  min- 
erals. The  three,  therefore,  are  placed  in  the  same  group,  in  which 
case  all  must  be  regarded  as  salts  of  metallic  acids,  thus:  Ti02  =  TiTiO*, 
Sn02=SnSn04,  zircon =ZrSi04.  Other  authorities  regard  zircon  as  an 
isomorphous  mixture  of  Ti02  and  Si02.  In  this  book  zircon  is  placed 
with  the  silicates  and  the  other  minerals  are  considered  as  oxides. 

The  two  manganese  dioxides  are  polianUe  and  pyrolusite.  The  former 
is  tetragonal  and  the  latter  orthorhombic.  It  is  possible,  however,  that 
the  crystals  of  pyrolusite  are  pseudomorphs  and  that  the  substance  is  a 
mixture  of  polianite  and  some  hydroxide,  as  it  nearly  always  contains 
about  2  per  cent  H2O. 

The  three  titanium  oxides  are  rutile,  which  is  tetragonal;  brookite, 
which  is  orthorhombic,  and  analase  or  octahedrite,  which  is  tetragonal. 
Although  rutile  and  anatase  crystallize  in  the  same  system,  their  axial 
ratios  are  different,  as  are  also  their  crystal  habits  and  their  physical 
properties.    A  few  of  these  differences  are  indicated  below: 

Rutile     a  :  c=i  :    .6439;    Sp.  Gr.  =4.283;   00^=2.6158;    em— 2.9029. 
Anatase         =1:1.7771;    Sp.  Gr.  =3.9     ;   a>,,a=2.56i8;    €«*=  2.4886. 


168  DESCRIPTIVE  MINERALOGY 

Of  the  three  modifications  of  titanium  dioxide,  anatase  may  be 
made  at  a  comparatively  low  temperature.  Brookite  requires  a  higher 
temperature  for  its  production,  but  rutile  is  producible  at  both  high 
and  low  temperatures.  Under  the  conditions  of  nature  both  brookite 
and  anatase  pass  readily  into  rutile. 

Of  the  seven  dioxides  discussed,  four  are  members  of  a  single  group. 

RUTILE  GROUP 

The  rutile  group  consists  of  four  minerals  apparently  completely 
isomorphous,  though  no  mixed  crystals  of  any  two  have  been  discovered.1 
All  crystallize  in  the  tetragonal  system  (ditetragonal  bipyramidal  class), 
with  the  same  forms  and  with  closely  corresponding  axial  ratios.  The 
names  of  the  members  of  the  group  and  their  axial  ratios  follow: 

Cassiterile  (Sn02)  ale  =  i  :  .6726 

Rutile        (Ti02)  =  1  :  .6439 

Polianite   (Mn02)  =1  •  .6647 

PlattnerUe  (PbOa)  - 1  :  .6764 

Cassiterite  (S11O2) 

Cassiterite,  or  tinstone,  is  the  only  worked  ore  of  tin.  It  occurs  as 
rolled  pebbles  of  a  dark  brown  color  in  the  beds  of  streams,  as  fibrous 
aggregates,  and  as  glistening  black  crystals  associated  with  other  min- 
erals in  veins. 

The  analyses  of  cassiterite  indicate  it  to  be  essentially  an  oxide  of 
tin,  or,  possibly,  a  stanyl  stannate  ((SnO)Sn03),  with  the  composition, 
Sn=78.6  per  cent;  0=21.4  per  cent.  The  mineral  nearly  always  con- 
tains some  iron  oxide  and  often  oxides  of  tantalum,  of  zinc  or  of  arsenic. 
The  presence  of  iron  and  tantalum  is  so  general  that  most  crystals  of 
cassiterite  may  be  regarded  as  isomorphous  mixtures  of  (SnO)(Sn03), 
Fe(SnOs)  and  Fe(Ta03)2.  Thus,  a  crystal  from  the  Etta  Mine  in  the 
Black  Hills,  S.  D.,  gave  Sn02=9436;  FeO=i.62;  Ta205=2.42  and 
Si02=i.oo,  indicating  a  mixture  of  5  pts.  of  Fe(Ta03)2,  18  pts.  of 
Fe(Sn03)  and  303.5  pts.  of  (SnO)(Sn03). 

The  crystals  of  cassiterite  have  an  axial  ratio  of  1  :  .6726.  They  are 
usually  short  prisms  in  habit.  They  often  consist  of  the  simple  com- 
bination P(ni)  and  P  00(101)  (Fig.  77),  or  of  these  forms,  together 
with  3PI (321)  and  various  prisms  (Fig.  78).    Twins  are  common,  the 

1  An  isomorphous  mixture  of  the  rutile  and  cassiterite  molecules  has  recently 
been  described  from  Greifenstein,  Austria,  but  its  existence  has  not  yet  been  con- 
firmed. 


OXIDES  169 

twinning  plane  being  P  oo  (101).  When  the  individuals  twinned  have 
small  prismatic  faces  the  resulting  combination  is  often  called  a  visor 
twin  (Fig.  79),  because  of  its  supposed  resemblance  to  the  visor  of  a 
helmet.  By  repetition  of  the  twinning  very  complex  groupings  are 
produced.    The  angle  m  A  *  ■'  ™  5&°  i*)'- 


Fio.  77. 
Fio.  77.— Cassiterite  Crystal  with  P, 
Fio.  7S. — Cassiterite  Crystal  with  s,  t  and  «  P,  1 1 


Fig.  78. 
(1)  and  P  «  ,  101  («). 

k>W;  JP|,3«M. 


(m) 


The  cleavage  of  cassiterite  is  imperfect  parallel  to  on  p  00  (100)  and 
P(m).  Its  fracture  is  uneven.  The  color  of  the  massive  mineral  is 
some  dark  shade  of  brown  by  reflected  light,  and  of  the  crystals  black. 
By  transmitted  light,  the  mineral  is  brown  or  black.  Its  luster  is  very 
brilliant,  and  its  streak  is  white,  gray  or  brown.    The  purest  specimens 


Fig.  79. — Cassiterite  Twinned  about  P 


Visor  Twin. 


are  nearly  transparent,  though  the  ordinary  varieties  are  opaque.  Their 
hardness  is  about  6.5  and  density  about  7.  The  mineral  is  a  noncon- 
ductor of  electricity.  Its  refractive  indices  for  yellow  light  are:  «=  1.9965, 
(=2.0931. 

Three  varieties  of  cassiterite  are  recognized,  distinguished  by  physical 
characteristics.     The  ordinary  variety  known  as  tinstone  is  crystallized 


170  DESCRIPTIVE  MINERALOGY 

or  massive.  Wood  tin  is  a  botryoidal  or  reniform  variety,  concentric  in 
structure  and  composed  of  radiating  fibers.  The  third  variety  is  stream 
tin.  This  consists  of  water-worn  pebbles  found  in  the  beds  of  streams 
that  flow  over  cassiterite-bearing  rocks. 

Cassiterite  is  only  slightly  acted  upon  by  acids.  It  may  be  reduced 
to  a  metallic  globule  of  tin  only  with  difficulty,  even  when  mixed 
with  sodium  carbonate  and  heated  intensely  on  charcoal.  With 
borax  it  yields  slight  reactions  for  iron,  manganese  or  other  impurities. 
When  placed  in  dilute  hydrochloric  acid  with  pieces  of  granulated  zinc, 
fragments  of  cassiterite  become  covered  with  a  dull  gray  coating  of 
metallic  tin  which  can  be  burnished  by  rubbing  with  a  cloth  or  the  hand. 
When  rubbed  by  the  hand  the  odor  of  tin  in  contact  with  flesh  is  easily 
detected. 

The  mineral  is  most  easily  distinguished  from  other  compounds  that 
resemble  it  in  appearance  by  its  high  density  and  its  inertness  when 
treated  with  reagents  or  before  the  blowpipe. 

Syntheses. — Crystals  of  cassiterite  have  been  obtained  by  passing 
steam  and  vapor  of  tin  chloride  or  tin  fluoride  through  red-hot  porcelain 
tubes,  and  by  the  action  of  tin  chloride  vapor  upon  lime. 

Occurrence  and  Origin. — Tinstone  is  found  as  a  primary  mineral  in 
coarse  granite  veins  with  topaz,  tourmaline,  fluorite,  apatite  and  a  great 
number  of  other  minerals.  It  also  occurs  impregnating  rocks,  sometimes 
replacing  the  minerals  of  which  they  originally  consisted.  In  these 
cases  it  is  the  product  of  pneumatolytic  processes.  In  many  places  it 
constitutes  a  large  proportion  of  the  gravel  in  the  beds  of  streams. 

Localities  and  Production. — The  crystallized  mineral  occurs  at  many 
places  in  Bohemia  and  in  Saxony,  at  Limoges  in  France  and  sparingly 
in  a  few  places  in  the  United  States,  especially  near  El  Paso,  Texas, 
in  Cherokee  Co.,  N.  C,  in  Lincoln  Co.,  S.  C,  and  near  Hill  City,  S.  D. 
Massive  tinstone  and  stream  tin  occur  in  laige  enough  quantities  to  be 
mined  in  Cornwall,  England;  on  the  Malay  Peninsula  and  on  the  islands 
lying  off  its  extremity;  in  Tasmania;  in  New  South  Wales,  Victoria 
and  Queensland,  Australia;  in  the  gold  regions  of  Bolivia;  at  Durango 
in  Mexico,  and  at  various  points  in  Alaska,  at  some  of  which  there 
are  400  lb.  of  cassiterite  in  a  cubic  yard  of  gravel. 

The  principal  tin  ore-producing  regions  of  the  world  are  the  Straits 
district,  including  the  Malay  Peninsula  and  the  islands  of  the  Malay 
Archipelago;  Australia;  Cornwall,  England;  the  Dutch  East  Indies,  and 
Bolivia.  Of  the  total  output  of  122,752  tons  of  tin  produced  in  1911, 
61,712  tons  were  made  from  the  Straits  ore,  25,312  tons  from  the  ore 
produced  in  Bolivia  and  16,800  tons  from  Banka  ore.    Of  the  total 


OXIDES  171 

quantity  of  tin  produced  about  78  per  cent  is  said  to  come  from  stream 
tin  and  22  per  cent  from  ore  obtained  from  veins.  The  quantity 
obtained  from  ore  mined  in  the  United  States  in  191 1  included  61  tons 
from  Alaskan  stream  tin  and  two  tons  from  the  tinstone  mined  in  the 
Franklin  Mountains  near  El  Paso,  Texas.  Mines  have  been  opened  in 
San  Bernardino  Co.,  California,  and  in  the  Black  Hills,  South  Dakota, 
but  they  have  not  proved  successful.  The  mines  at  El  Paso,  Texas,  are 
not  yet  fully  developed,  although  they  promise  to  be  profitable  in  the 
near  future.  The  crystals  are  scattered  through  quartz  veins  and 
through  a  pink  granite  near  the  contacts  with  the  veins.  The  average 
composition  of  the  ore  is  2  per  cent.  This  is  concentrated  to  a  60  per 
cent  ore  before  being  smelted.  The  production  during  1912  was  130 
tons  of  stream  tin  from  Buck  Creek,  Alaska.  This  was  valued  at 
$124,800.  In  the  following  year  3  tons  of  cassiterite  were  shipped  from 
Gaffney,  S.  C.  The  imports  of  tin  into  the  United  States  during  191 1 
were  53,527  tons  valued  at  more  than  $43,300,000. 

Extraction. — The  tin  is  extracted  from  the  concentrated  ore  by  the 
simple  process  of  reduction.  Alternate  layers  of  the  ore  and  charcoal 
are  heated  together  in  a  furnace,  when  the  metal  results.  This  collects 
in  the  bottom  of  the  furnace  and  is  ladled  or  run  out.  The  crude  metal  is 
refined  by  remelting  in  special  refining  furnaces. 

Uses  of  the  Metal, — The  metal  tin  is  employed  principally  for  coating 
other  metals,  either  to  prevent  rusting  or  to  prevent  the  action  upon 
them  of  chemical  reagents.  Tmplate  is  thin  sheet  iron  covered  with 
tin.  Copper  for  culinary  purposes  is  also  often  covered  with  this  metal. 
It  is  used  also  extensively  in  forming  alloys  wifj^fnpppi^  antjn)^pyj 
bismuth  and  lead.  Among  the  most  important  of  these  alloys  are 
bronze,  bell  metal,  babbitt  metal,  gun  metal,  britannia,  pewter  and  soft 
solder.  Its  alloy,  or  amalgam,,  with  mercury  is  used  in  coating  mirrors. 
Several  of  its  compounds  also  find  uses  in  the  arts.  Tin  oxide  is  an  im- 
portant constituent  of  certain  enamels.  The  chlorides  are  used  exten- 
sively in  dyeing  calicoes,  and  the  bisulphide  constitutes  "  bronze 
powder  "  or  "  mosaic  gold,"  a  powder  employed  for  bronzing  plaster, 
wood  and  metals. 

Ruffle  (Ti02) 

Rutile  is  one  of  the  oxides  of  the  comparatively  rare  element  titanium. 
It  occurs  commonly  in  dark  brown  opaque  cleavable  masses  and  in  bril- 
liant black  crystals. 

Pure  rutile  consists  of  40  per  cent  O  and  60  per  cent  Ti.  Nearly  all 
specimens,  however,  contain  in  addition  some  iron,  occasionally  as  much 


172 


DESCRIPTIVE  MINERALOGY 


as  9  per  cent  or  10  per  cent,  which  is  probably  due  to  the  admixture  of 
Fe203  and  FeTiOa  in  solid  solution. 

Rutile  is  peifectly  isomorphous  with  cassiterite.  Its  axial  ratio  is 
i  :  .6439.  The  principal  planes  observed  on  its  crystals  are  practically 
the  same  as  those  observed  on  cassiterite  (Fig,  80).  Twins  are  common, 
with  Poo  (roi)  the  twinning  plane.  (Fig.  81.)  This  twinning  is  often 
repeated,  producing  elbow-shaped  groups  (Fig.  8a),  or  by  further  repe- 


Fig.  80.— Rutile  Crystals  with  «*P,  ito(m);  »P»,  100(0);  P« 
«  P3,  310  (0;  P3,  313  CO  wd  3PI.  3»  W- 


Fig.  8c— Rutile  Eightling  Twinned  about  P  °o  (101). 
Fig.  81.— Rutile  Twinned  about  P«  (101).     Elbow  Twin. 


tition  wheel-shaped  aggregates  (Fig.  83).  In  another  common  law  the 
twinning  plane  is  3P 00  (301)  (Fig.  84).  The  angle  111  ^ili  =  $6"  52J'. 
The  crystals  are  prismatic  and  even  sometimes  acicular  in  habit.  Their 
prismatic  planes  are  vertically  striated. 

The  cleavage  of  rutile  is  quite  distinct  parallel  to  00  P(no)  and  less 
so  parallel  to  00  P  00  (100). 

The  mineral  is  reddish  brown,  yellowish  brown,  black  or  bluish 
brown  by  reflected  light  and  sometimes  deep  red  by  transmitted  light. 
Many  specimens  are  opaque  but  some  are  translucent  to  transparent. 


OXIDES 


173 


The  latter  are  often  pleochroic  in  tints  varying  between  yellow  and 
blood-red.  The  streak  is  pale  brown.  The  hardness  of  the  mineral  is 
6  to  6.5  and  its  density  about  4.2.  It  is  an  electric  nonconductor  at 
ordinary  temperatures.  Its  refractive  indices  for  yellow  light  are: 
40=2.6030,  €=2.8894. 

Rutile  is  infusible  and  insoluble.  Its  reactions  with  beads  of  borax 
and  microcosmic  salt  are  usually  obscured  by  the  iron  present.  When 
this  metal  is  present  only  in  small  quantities  the  microcosmic  salt  bead 
is  colorless  while  hot,  but  violet  when  cold,  if  it  has  been  heated  for  some 
time  in  the  reducing  flame  of  the  blowpipe. 

The  most  characteristic  chemical  reaction  of  rutile  is  obtained  upon 
fusing  it  with  sodium  carbonate  on  charcoal,  dissolving  the  fused  mass  in 


Fig.  83.  Fig.  84. 

Fig.  83.— Rutile  Cyclic  Sixling  Twinned  about  P  «>  (101). 

Fig,  84. — Rutile  Twinned  about  3P  <*>  (301).    Elbow  Twin.    Forms:  °o  P2,  210  (A), 

and  Poo|  101  (<?). 


an  excess  of  hydrochloric  acid  and  adding  to  the  solution  small  scraps  of 
tin.  Upon  heating  for  some  little  time,  the  solution  assumes  a  violet 
color.     This  is  a  universal  test  for  the  metal  titanium. 

Some  of  the  dark  red  and  reddish  brown  massive  varieties  of  rutile 
may  be  confounded  with  some  varieties  of  garnet,  which,  however,  are 
much  harder.  Its  density,  its  infusibility  and  the  reaction  for  titanium 
serve  to  characterize  the  mineral  perfectly. 

Pseudomorphs  of  rutile  after  hematite  and  after  brookite  and  ana- 
tase  have  been  described.     It  often  changes  into  ilmenite  and  sphene. 

Syntheses. — By  the  reaction  between  TiCU  and  water  vapcr  in  a  red- 
hot  porcelain  tube,  crystals  of  rutile  are  formed.  Twins  are  produced 
by  submitting  precipitated  titanic  acid  in  a  mass  of  molten  sodium  tung- 
state  to  a  temperature  of  10000  for  several  weeks. 

Occurrence  and  Origin. — Rutile  is  often  found  as  crystals  embedded 
in  limestone  and  in  the  quartz  or  feldspar  of  granite  and  other  igneous 


* 


174  DESCRIPTIVE  MINERALOGY 

rocks,  as  long  acicular  crystals  in  slates,  and  as  grains  in  the  rock  known 
as  nelsonite.  It  occurs  also  as  fine  hair-like  needles  penetrating  quartz, 
forming  the  ornamental  stone  "  fteches  d'amour,"  and  as  grains  in  the 
gold-bearing  sand  regions.  When  primary  it  is  probably  always  a 
product  of  magmatic  processes,  either  crystallizing  from  a  molten  magma 
or  being  the  result  of  pneumatolysis. 

Localities. — Handsome  crystals  of  the  mineral  occur  at  Arendal,  in 
Norway;  in  Tyrol,  and  at  St.  Gothard  and  in  the  Binnenthal,  Switzer- 
land. In  the  United  States  large  crystals  have  been  obtained  at  Barre, 
Mass.;  at  Sudbury,  Chester  Co.,  Penn.;  at  Stony  Point,  Alexander  Co., 
N.  C;  at  Graves  Mt.,  in  Georgia;  at  Magnet  Cove,  in  Arkansas,  and 
in  Nelson  Co.,  Va.  In  the  latter  place  it  occurs  in  large  quantity  as 
crystals  disseminated  through  a  coarse  granite  rock.  The  rock  con- 
taining about  10  per  cent  of  rutile  is  mined  as  an  ore.  It  constitutes  the 
principal  source  of  the  mineral  in  the  United  States.  A  second  type 
of  occurrence  in  the  same  locality  is  a  dike-like  rock,  nelsonite,  composed 
of  ilmenite  and  apatite,  in  which  the  ilmenite  is  in  places  almost 
completely  replaced  by  rutile. 

Uses. — The  mineral  is  not  of  great  economic  importance.  It  is  used 
in  small  quantity  to  impart  a  yellow  color  to  porcelain  and  to  give  an 
ivory  tint  to  artificial  teeth.  It  is  also  used  in  the  manufacture  of  the 
alloy  ferro-titanium  which  is  added  to  steel  to  increase  its  strength. 
Recently  the  use  of  titaniferous  electrodes  in  arc  lights,  and  the  use  of 
titanium  for  filaments  in  incandescent  lamps  have  been  proposed.  Some 
of  the  salts  of  titanium  are  used  as  dyes  and  others  as  mordants.  Most 
of  the  ferro-titanium  made  in  the  United  States  is  manufactured  from 
titaniferous  magnetite. 

Production. — The  only  rutile  mined  in  the  United  States  during  19 13 
came  from  Roseland,  Nelson  Co.,  Virginia.  It  amounted  to  305  tons  of 
concentrates  containing  about  82  per  cent  Ti02.  At  the  same  time  there 
were  separated  about  250  tons  of  ilmenite  (see  p.  462) 

Polianite  (MnC>2)  is  usually  in  groups  of  tiny  parallel  crystals  and 
as  crusts  of  crystals  enveloping  crystals  of  manganite  (MnO  •  OH).  Their 
axial  ratio  is  1  :  .6647.  The  color  of  the  mineral  is  iron-gray.  Its  streak 
is  black,  its  hardness  6-6.5  and  density  4.99.  It  dissolves  in  HC1  evolv- 
ing chlorine.  It  is  distinguished  from  pyrdusile  by  its  greater  hardness 
and  its  lack  of  water.  The  mineral  is  extremely  rare,  being  found  in 
measurable  crystals  only  at  Platten  in  Bohemia.  It  occurs  in  pseudo- 
morphs  after  manganite  at  a  number  of  other  points  in-  Europe  and  at  a 
few  points  elsewhere,  but  in  most  cases  it  has  not  been  clearly  distin- 


OXIDES  175 

guished  from  pyrolusite.  The  rarity  of  its  crystals  is  regarded  by  some 
mineralogists  as  being  due  to  the  fact  that  in  most  of  its  occurrences 
polianite  is  colloidal  (a  gel). 

Plattnerite  (Pb02)  is  usually  massive,  but  it  occurs  in  prismatic 
crystals  near  Mullan  in  Idaho.  Their  axial  ratio  is  i  :  .6764.  They  are 
usually  bounded  by  00  P  00  (100),  3P  00  (301),  P  00  (101),  oP  (001)  and 
often  fP(332).  The  mineral  is  found  also  in  crusts.  Its  color  is 
iron-black  and  its  streak  chestnut-brown.  Its  hardness  is  5-5.5  and 
its  density  8.6.  It  is  brittle  and  is  easily  fusible  before  the  blow- 
pipe, giving  off  oxygen  and  coloring  the  flame  blue.  It  yields  a  lead 
bead.  It  is  difficultly  soluble  in  HNO3,  but  easily  soluble  in  HC1  with 
evolution  of  chlorine.  Plattnerite  is  found  at  Leadhills  and  at  Wanlock- 
head,  Scotland,  and  at  the  "  As  You  Like  "  Mine  near  Mullan,  Idaho. 


Pyrolusite  (Mn02) 


Pyrolusite  is  often  the  result  of  the  alteration  of  the  hydroxide,  man- 
ganite,  or  of  polianite.  The  few  measurable  crystals  that  have  been 
studied  seem  to  indicate  that  their  form  is  pseudomorphic  after  the 
hydroxide.  The  change  by  which  manganite  may  pass  over  into  pyro- 
lusite is  represented  by  the  reaction  2MnO(OH)+0=2Mn02+H20. 
Pyrolusite  may  be,  however,  only  a  slightly  hydrated  form  of  polianite. 

An  analysis  of  a  specimen  from  Negaunee,  Mich.,  gave: 


MnO 

0 

CaO 

BaO 

Si02 

Limonite 

H2O 

Total 

79.46 

17.48 

.18 

■38 

.18 

•31 

I.94 

99*93 

Pyrolusite,  as  usually  found,  is  in  granular  or  columnar  masses,  or  in 
masses  of  radiating  fibers.  It  is  a  soft,  black  mineral  with  a  hardness  of 
only  2  or  2.5  and  a  density  of  about  4.8.  Its  luster  is  metallic  and  its 
streak  black.    It  is  a  fairly  good  conductor  of  electricity. 

The  reactions  of  this  mineral  are  practically  the  same  as  those  of 
polianite  and  manganite  (see  p.  191),  except  that  only  a  small  quantity 
of  water  is  obtained  from  it  by  heating.  Upon  strong  heating  it  yields 
oxygen,  according  to  the  equation  3Mn02=Mn3+302. 

The  manganese  minerals  are  easily  distinguished  from  other  minerals 
by  the  violet  color  they  give  to  the  borax  bead  and  by  the  green  product 
obtained  when  they  are  fused  with  sodium  carbonate.  Pyrolusite  is 
distinguished  from  manganite  by  its  physical  properties,  and  from  poli- 
anite by  its  softness. 


176  DESCRIPTIVE  MINERALOGY 

Localities. — Pyrolusite  is  worked  at  Elgersberg,  near  Iimenau  in 
Thuringia;  at  Vorder  Ehrensdorf  in  Moravia;  at  Platten  in  Bohemia; 
at  Cartersville,  Ga.;  at  Batesville,  Ark.,  and  in  the  Valley  of  Virginia. 
A  manganiferous  silver  ore  containing  considerable  quantities  of  pyro- 
lusite is  mined  in  the  Leadville  district,  Colorado,  and  large  quan- 
tities of  manganiferous  iron  ores  are  obtained  in  the  Lake  Superior 
region. 

Uses. — Pyrolusite,  together  with  the  other  manganese  ores  with 
which  it  is  mixed,  is  the  source  of  nearly  all  the  manganese  compounds 
employed  in  the  arts.  Some  of  the  ores,  moreover,  are  argentiferous 
and  others  contain  zinc.  From  these  silver  and  zinc  are  extracted.  The 
most  important  use  of  the  mineral  is  in  the  iron  industry.  In  this  indus- 
try, however,  much  of  the  manganese  employed  is  obtained  from  man- 
ganiferous iron  ores.  The  alloys  spiegeleisen  and  ferro-manganese  are 
employed  very  largely  in  the  production  of  an  iron  used  in  casting  car 
wheels.  It  is  extremely  hard  and  tough.  The  manganese  minerals  are 
also  used  in  glass  factories  to  neutralize  the  green  color  imparted  to  glass 
by  the  ferruginous  impurities  in  the  sands  from  which  the  glass  is  made. 
They  are  also  used  in  giving  black,  brown  and  violet  colors  to  pottery 
and  some  of  their  salts  are  valuable  mordants.  Pyrolusite,  finally,  is  the 
principal  compound  by  the  aid  of  which  chlorine  and  oxygen  are  pro- 
duced. 

Production. — The  United  States  in  191 2  produced  about  1,664  tons 
of  manganese  ores,  valued  at  $15,723,  and  all  came  from  Virginia,  South 
Carolina  and  California.  In  previous  years  the  ores  had  been  mined 
also  in  Arkansas,  Tennessee  and  Utah.  Moreover,  there  were  imported 
into  the  country  300,661  tons,  valued  at  $1,769,000.  Nearly  all  of  this 
was  used  in  the  manufacture  of  spiegeleisen.  The  domestic  product  was 
used  in  the  chemical  industries  largely  in  the  manufacture  of  manganese 
brick.  Of  the  manganiferous  iron  ores  about  818,000  tons  were  produced 
in  191 2.  These  were  utilized  mainly  as  ores  of  iron,  though  a  large  por- 
tion was  used  as  a  flux.  The  product  of  manganiferous  silver  ores  aggre- 
gated about  48,600  tons,  all  of  which  was  used  as  a  flux  for  silver-lead 
ores.  Nearly  all  of  this  came  from  Colorado.  In  addition  there  were 
imported  iron-manganese  alloys  valued  at  $3,935,000. 

Anatase  and  Brookite  (Ti02)^ 

As  has  already  been  stated,  the  compound  Ti02  is  trimorphous,  one 
form  being  orthorhombic  and  the  two  others  tetragonal.  Of  the  latter, 
one  has  already  been  described  as  rutile.  The  other  is  anatase,  or  octa- 
hedrite.    The  orthorhombic  form  is  known  as  brookite.    Anatase  and 


OXIDES  177 

rutile  are  separated  because  of  the  difference  in  their  axial  ratios  and  in 
the  habits  of  their  crystals.  Both  are  ditetragonal  bipyramidal,  but 
a:c  for  rutile  is  i  :  .6439  and  for  anatase  1  :  1.7771.  Brookite  is 
orthorhombic  bipyramidal  with  alb:  £=.8416  :  1  :  .9444. 

Both  anatase  and  brookite  have  the  same  empirical  composition, 
which  is  similar  to  that  of  rutile. 

Crystals  of  anatase  are  usually  sharp  pyramidal  with  the  form  P(in) 
predominating  (Fig.  85),  blunt  pyramidal  with  $P(ii3)  or  |P(ii7) 
predominating  (Fig.  86),  or  tabular  parallel  to  oP(ooi).  Twins  are 
common  in  some  localities,  with  P  00  (101)  the  twinning  plane.  The 
angle  111 A  iTi  =  82°  91'. 

The  mineral  is  colorless  and  transparent,  or  dark  blue,  yellow,  brown 


Fig.  85.  Fig.  86. 

Fig.  85. — Anatase  Crystal  with  P,  ui'(^). 
Fig.  86. — Anatase  Crystal  with  JP,  113  (s);  P,  in  (/>);  IP,  117  (»);    00 P,  no  (w); 

00  P  00  f  100  (a)  and  P  00  ,  101  (e). 

or  nearly  black  and  almost  opaque.  Its  streak  is  colorless  to  light 
yellow.  Its  cleavage  is  perfect  parallel  to  P  and  oP  and  its  fracture 
conchoidal.  Its  hardness  is  between  5  and  6  and  its  density  is  3.9.  This 
increases  to  4.25  upon  heating  to  a  red  heat,  possibly  due  to  its  partial 
transformation  into  rutile.  The  mineral  is  insoluble  in  acids  except 
hot  concentrated  H2SO4.  It  is  a  nonconductor  of  electricity.  Its 
indices  of  refraction  for  yellow  light  are:  a>=  2.5618,  c=  2.4886. 

Brookite  crystals  are  usually  tabular  parallel  to  00  P  66  (100)  and 
elongated  in  the  direction  of  the  c  axis.  Nearly  all  crystals  are 
striated  in  the  vertical  zone.  Although  many  forms  have  been  identi- 
fied on  them,  by  far  the  most  common  is  P2(i22).  In  some  cases  this  is 
the  only  pyramidal  form  present,  as  in  the  type  known  as  arkansite 
(Fig.  87).  Twins  are  rare,  with  ooP2~(2io)  the  twinning  plane.  The 
angle  in  A  ili  =  64°  17'. 


178 


DESCRIPTIVE  MINERALOGY 


Brookite  may  be  opaque,  translucent  or  transparent.  Its  color 
varies  from  yellowish  brown,  through  brownish  red,  to  black  (arkansite). 
Its  streak  is  brownish  yellow.  Its  cleavage  is  imperfect  parallel  to 
oo Poo  (101),  and  its  fracture  uneven  or  conchoidal.  Its  hardness  is 
5-6  and  density  about  4.  Upon  heating  its  density  increases  to  that  of 
rutile.  Its  refractive  indices  for  yellow  light  are:  a=  2.5832,  0=  2.5856, 
7=  2.7414.    It  fuses  at  about  15600,  and  is  insoluble  in  acids. 

The  chemical  properties  of  both  brookite  and  anatase  are  similar 
to  those  of  rutile.  They  are  distinguished  from  rutile  by  their  physical 
properties  and  their  crystallization. 

Both  brookite  and  anatase  alter  to  rutile. 

Syntheses. — Upon  heating  TiF4  with  water  vapor  at  a  temperature 


z 


m 


m 


Fio.  87. — Brookite  Crystals  with   00  P,  no  (m);   JP,  112  (2)  and  PT,  122  (<?).    The 

combination  m  and  e  is  characteristic  for  Arkansite. 


below  that  of  vaporizing  cadmium,  crystals  of  anatase  are  produced. 
If  the  temperature  is  raised  above  the  point  of  vaporization  of  cadmium 
and  kept  below  that  of  zinc,  crystals  of  brookite  result. 

Occurrence. — Brookite  and  anatase  occur  as  crystals  on  the  walls  of 
clefts  in  crystalline  silicate  rocks  and  in  weathered  phases  of  volcanic 
rocks.  They  are  mainly  pneumatolytic  products,  the  production  of  the 
one  or  the  other  depending  upon  the  temperature  at  which  the  Ti(>2  was 
deposited. 

Localities. — Fine  brookite  crystals  are  found  at  St.  Gothard,  in 
Switzerland;  at  Pregrattan,  in  the  Tyrol;  near  Tremadoc,  in  Wales; 
at  Miask,  in  Russia,  and  at  Magnet  Cove,  Arkansas. 

Anatase  crystals  are  less  common  than  those  of  brookite  but  they 
occur  at  many  points  in  Switzerland,  especially  in  the  Binnenthal; 
near  Bourg  d'Oisans,  France;  at  many  points  in  the  Urals,  Russia;  in 
the  diamond  fields  of  Brazil,  and  at  the  brookite  occurrences  in  Arkansas. 


CHAPTER  VIII 
THE  HYDROXIDES 

The  hydroxides,  as  has  already  been  explained,  may  be  looked  upon 
as  derivatives  of  water,  in  which  only  a  portion  of  the  hydrogen  has  been 
replaced.  The  group  includes  several  minerals  of  economic  importance, 
among  which  is  the  fine  gem  mineral  opal.  All  the  hydroxides  yield 
water  when  heated  in  a  glass  tube,  but  they  do  not  yield  it  as  readily  as 
do  salts  containing  water  of  crystallization. 

A  few  of  the  hydroxides  may  act  as  acids  forming  salts  with  metals. 
Diaspore,  for  instance,  is  an  hydroxide  of  aluminium  AlO-OH,  or 

/O— H 

A1C  ,  which  appears  to  be  able  to  form  salts;  at  least,  the  chemical 

X) 

composition  of  some  of  the  members  of  an  important  group  of  minerals, 
the  spinels,  may  be  explained  by  regarding  them  as  salts  of  this  acid 
(see  p.  195). 

Opal  (Si02+Aq) 

The  true  position  of  opal  in  the  classification  of  minerals  is  somewhat 
doubtful.  From  the  analyses  made  it  appears  to  be  a  combination  of 
amorphous  silica  and  water,  or,  perhaps,  a  mixture  of  silica  in  some  form 
and  a  hydroxide  of  silicon.  The  percentage  of  water  present  is  variable. 
In  some  specimens  it  is  as  low  as  3  per  cent,  while  in  others  it  is  as  high 
as  13  per  cent.  The  mineral  is  not  known  in  crystals.  It  is  probably  a 
colloid,  in  which  the  water  is,  in  part  at  least,  mechanically  held  in  a  gel 
of  Si02.  It  occurs  only  in  massive  form,  in  stalactitic  or  globular  masses 
and  in  an  earthy  condition. 

When  pure  the  mineral  is  colorless  and  transparent.  Usually,  how- 
ever, it  is  colored  some  shade  of  yellow,  red,  green  or  blue,  when  it  is 
translucent  or  sometimes  even  opaque.  The  red  and  yellow  varieties  con- 
tain iron  oxides  and  the  green,  pros  opal,  some  nickel  compound.  The 
play  of  color  in  gem  opal  is  due  to  the  interference  of  light  rays  reflected 
from  the  sides  of  thin  layers  of  opal  material  with  different  densities 
from  that  of  the  main  mass  of  the  mineral  they  traverse.  The  hardness 
of  opal  is  5.5-6.5  and  its  density  about  2.1.  Its  refractive  index  for 
yellow  light,  »=  1.4401.    It  is  a  nonconductor  of  electricity. 

179 


180  DESCRIPTIVE  MINERALOGY 

The  principal  varieties  of  opal  are: 

Precious  opal,  a  transparent  variety  exhibiting  a  delicate  play  of 
colors, 

Fire  opal,  a  precious  opal  in  which  the  colors  are  quite  brilliant 
shades  of  red  and  yellow, 

Girasol,  a  bluish  white  translucent  opal  with  reddish  reflections, 

Common  opal,  a  translucent  variety  without  any  distinct  play  of 
colors, 

Cachalong,  an  opaque  bluish  white,  porcelain-like  variety, 

Hyalite,  a  transparent,  colorless  variety,  usually  in  globular  or 
botryoidal  masses,  and 

Siliceous  sinter,  white,  translucent  to  opaque  pulverulent  accumula- 
tions and  hard  crusts,  deposited  from  the  waters  of  geysers  and  other 
hot  springs. 

Tripolite  and  infusorial  earth  are  pulverulent  forms  of  silica  in  which 
opal  is  an  important  constituent.  Tripoli  is  a  light  porous  siliceous 
rock,  supposed  to  have  resulted  from  the  leaching  of  calcareous  material 
from  a  siliceous  limestone.  Infusorial  earth  represents  the  remains  of 
certain  aquatic  forms  of  microscopic  plants  known  as  diatoms. 

Flint  and  Chert  are  mixtures  of  opal,  chalcedony  and  quartz. 

All  varieties  of  opal  are  infusible  and  all  become  opaque  when  heated. 
When  boiled  with  caustic  alkalies  some  varieties  dissolve  easily,  while 
others  dissolve  very  slowly. 

Syntheses. — Coatings  of  material  like  opal  have  been  noted  in  glass 
flasks  containing  hydrofluosilicic  acid  that  had  not  been  opened  for 
several  years.  Opal  has  also  been  obtained  by  the  slow  cooling  of  a 
solution  of  silicic  acid  in  water. 

Occurrence. — The  mineral  occurs  as  deposits  around  hot  springs. 
It  also  forms  veins  in  volcanic  rocks  and  is  embedded  in  certain  lime- 
stones and  slates,  where  it  is  probably  the  result  of  the  solution  of  the 
siliceous  spicules  and  shells  of  low  forms  of  life  and  subsequent  deposi- 
tion. It  also  results  from  the  solution  of  the  calcite  from  limestones 
containing  finely  divided  silica. 

It  is  not  an  uncommon  alteration  product  of  silicates.  It  seems  to 
have  been  deposited  from  both  cold  and  hot  water. 

Localities. — Precious  opal  is  found  near  Kashan,  in  Hungary;  at 
Zimapan,  Quaretaro,  in  Mexico;  in  Honduras;  in  Queensland  and 
New  South  Wales,  Australia,  and  in  the  Faroe  Islands.  Common  opal  is 
abundant  at  most  of  these  localities  and  is  found  also  in  Moravia, 
Bohemia,  Iceland,  Scotland  and  the  Hebrides.  Hyalite  occurs  in  small 
quantity  at  several  places  in  New  York,  New  Jersey,  North  Carolina, 


HYDROXIDES  181 

Georgia  and  Florida,  and  common  opal,  at  Cornwall,  Penn.,  and  in 
Calaveras  Co.,  California.  Common  opal  and  varieties  exhibiting  a  little 
fire  have  recently  been  explored  in  Humboldt  and  Lander  Counties, 
Nevada.  Siliceous  sinter  is  deposited  at  the  Steamboat  Springs  in 
Nevada  and  geyserite  (a  globular  form  of  the  sinter)  at  the  mouths  of  the 
geysers  in  the  Yellowstone  National  Park. 

Uses. — The  precious  and  fire  opals  are  popular  and  handsome  gems. 
Opalized  wood,  i.e.,  wood  that  has  been  changed  into  opal  in  such  a 
manner  as  to  retain  its  woody  structure,  is  often  cut  and  polished  for  use 
as  an  ornamental  stone.  Infusorial  earth,  a  white  earthy  deposit  of 
microscopic  shells  consisting  largely  of  opal  material,  possesses  many 
uses.  It  is  employed  in  the  manufacture  of  soluble  glass,  polishing 
powders,  cements,  etc.,  and  as  the  "  body,"  which,  saturated  with  nitro- 
glycerine, composes  dynamite.  Tripoli,  a  mixture  of  quartz  and  opal, 
is  used  as  a  wood  filler,  in  making  paint,  as  an  abrasive  and  in  the 
manufacture  of  filter  stones.  The  principal  sources  of  commercially 
valuable  opal  material  in  the  United  States  are  the  opalized  forest  in 
Apache  Co.,  Ariz.,  the  infusorial  earth  beds  at  Pope's  Creek  and  Dun- 
kirk, Md.,  various  places  in  Napa  Co.,  Cal.,  at  Virginia  City,  Nev., 
and  at  Drakesville,  N.  J.,  and  the  tripoli  beds  in  the  neighborhood  of 
Stella,  Mo.,  and  the  adjoining  portion  of  Illinois. 

Production. — The  total  quantity  of  infusorial  earth  and  tripoli  mined 
during  191 2  was  valued  at  $125,446.  The  aggregate  value  of  precious 
opal  obtained  in  191 2  was  $10,925.  This  came  from  California  and 
Arizona. 

Brucite  (Mg(OH)2) 

Brucite  is  the  hydroxide  of  magnesium.  It  is  a  white,  soft  mineral 
usually  occurring  in  crystals  or  in  foliated  masses. 

Analyses  of  the  mineral  correspond  very  closely  to  the  formula 
Mg(OH)2  which  requires  41.38  per  cent  Mg,  27.62  per  cent  O  and  31.00 
per  cent  H2O,  though  they  usually  show  the  presence  of  small  quantities 
of  iron  and  manganese.    A  specimen  from  Reading,  Perm.,  yielded: 


MgO 

Fe203 

MnO 

H20 

Total 

67.64 

.82 

■63 

30.92 

100.01 

The  crystallization  of  brucite  is  hexagonal  (ditrigonal  scalenohedral), 
a  :  c=i  :  1.5208.  The  crystals  are  tabular  in  habit  in  consequence  of 
the  broad  development  of  the  basal  plane  oP(oooi).  The  other  forms 
present  are  R(ioTi),  — 4R(o44i)  and  —  £R(oil3)  (Fig.  88).  The  angle 
ioTiai"ioi  =  97°38'. 


182  DESCRIPTIVE  MINERALOGY 

The  cleavage  of  brucite  is  very  perfect  parallel  to  oP(ooi),  and  folia 
that  may  be  split  off  are  flexible.  The  mineral  is  sectile.  Its  hardness 
is  2.5  and  its  density  2.4.  Its  color  is  white,  inclining  to  bluish  and 
greenish  tints,  and  its  luster  pearly  on  oP.    Brucite  is  transparent  to 

translucent.  It  is  pyroelectric  and  a  non- 
conductor of  electricity.  Its  refractive  indices 
for  red  light  are:  (0=1.559,  €=1.579. 

In  the  closed  tube  brucite,  like  other  hy- 
droxides, yields  water.    The  mineral  is  infusi- 
ble.   When  intensely  heated,  it  glows.    After 
Fig.  88.  —  Brucite  Crystal  heating,  it  reacts  alkaline.    When  moistened 
with  oR,  0001   W;  R   ^^  cobalt  nitIate  aoiution  and  heated,  it  turns 

a^-4R,  I*  it)!'       P^ the  characteristic  reaction  for  magnesium. 

The  pure  mineral  is  soluble  in  acids. 

Brucite  resembles  in  many  respects  gypsum,  talc,  diaspore  and  some 
micas.  It  is  distinguished  from  diaspore  and  mica  by  its  hardness  and 
from  talc  by  its  solubility  in  acids.  Gypsum  is  a  sulphate,  hence  the 
test  for  sulphur  will  sufficiently  characterize  it. 

Synthesis. — Crystals  have  been  made  by  precipitating  a  solution  of 
magnesium  chloride  with  an  alcoholic  solution  of  potash,  dissolving  the 
precipitate  by  heating  with  an  excess  of  KOH  and  allowing  to  cool. 

Occurrence  and  Origin. — Brucite  is  usually  associated  with  other 
magnesium  minerals.  It  is  often  found  in  veins  cutting  the  rock  known 
as  serpentine,  where  it  is  probably  a  weathering  product,  and  is  some- 
times found  in  masses  in  limestone,  especially  near  its  contact  with 
igneous  rocks. 

Localities. — It  occurs  crystallized  in  one  of  the  Shetland  Islands;  at 
the  Tilly  Foster  Iron  Mine,  Brewster,  N.  Y.;  at  Woods  Mine,  Texas, 
Penn.,  and  at  Fritz  Island,  near  Reading,  in  the  same  State. 

Gibbsite  (Al(OH)s) 

Gibbsite,  or  hydrargillite,  is  utilized  to  some  extent  as  an  ore  of  alu- 
minium. It  occurs  as  crystals,  in  granular  masses,  in  stalactites  and  in 
fibrous,  radiating  aggregates. 

Its  theoretical  composition  demands  65.41  per  cent  AI2O3  and 
34.59  per  cent  H2O.  Usually,  however,  the  mineral  is  mixed  with  bauxite 
(Al20(OH)4)  and  in  addition  contains  also  small  quantities  of  iron, 
magnesium,  silicon  and  often  calcium. 

Crystals  are  monoclinic  with  a  :  b  :  c=  1.709  :  1  :  1.918  and  £=85° 
29!'.    Their  habit  is  tabular.     Besides  the  basal  plane,  oP(ooi),  the 


HYDROXIDES  183 

two  most  prominent  forms  are  ooP*  (ioo)  and  ooP(no).  Thus  the 
plates  have  hexagonal  outlines.  They  have  a  perfect  cleavage  parallel 
to  the  base.    Twinning  is  common,  with  oP(ooi)  the  twinning  plane. 

The  mineral  has  a  glassy  luster  except  on  the  basal  plane  where  its 
luster  is  pearly.  It  is  transparent  or  translucent,  white,  pink,  green  or 
gray.  Its  streak  is  light,  its  hardness  is  2-3  and  specific  gravity  2.35. 
It  is  a  nonconductor  of  electricity.    Its  refractive  indices  are:  a=/3 

=  i-5347,  7=  1.5577- 

When  heated  before  the  blowpipe  the  mineral  exfoliates,  becomes 

white,  glows  strongly  but  does  not  fuse.  Upon  cooling  the  heated  mass 
is  hard  enough  to  scratch  glass,  The  mineral  dissolves  slowly  but  com- 
pletely in  hot  HC1  and  in  strong  H2SO4,  and  gives  a  blue  color  when 
moistened  with  Co(NOs)2  solution  and  heated. 

Gibbsite  resembles  most  closely  bauxite,  from  which  it  is  distin- 
guished principally  by  its  structure.  It  differs  from  wavellite  (p.  287), 
which  it  also  sometimes  resembles,  in  the  absence  of  phosphorus. 

Syntheses. — Crystals  of  gibbsite  have  been  made  by  heating  on  a 
water  bath  a  saturated  solution  of  Al(OH)3  in  dilute  ammonia  until  all 
of  the  ammonia  evaporates;  and  also  by  gradually  precipitating  the 
hydroxide  from  a  warm  alkaline  solution  by  means  of  a  slow  stream 
of  C02.  * 

Occurrence.^wThe  mineral  rarely  occurs  in  pure  form.  It  is  found  in 
veins  and  in  cavities  in  various  schistose  and  igneous  rocks.  It  is  prob- 
ably a  weathering  product  of  aluminous  silicates. 

Localities. — Gibbsite  has  been  reported  as  existing  in  small  quantities 
at  various  points  in  Europe,  near  Bombay,  India,  and  at  several  places 
in  South  America  and  Africa.  In  the  United  States  it  occurs  at  Rich- 
mond, Mass.,  at  Union  Vale,  Dutchess  Co.,  N.  Y.,  and  mixed  with 
bauxite  at  several  of  the  occurrences  of  this  mineral  (see  page  186). 

Uses, — It  is  mined  with  bauxite  as  a  source  of  aluminium. 

Limonite  (Fe403(OH)tt) 

Limonite  is  an  earthy  or  massive  reddish  brown  mineral  whose 
composition  and  crystallization  are  but  imperfectly  known.  It  is  an 
important  iron  ore  called  in  the  trade  "  brown  hematite." 

The  analyses  of  limonite. range  between  wide  limits,  largely  because 
of  the  great  quantities  of  impurities  mixed  with  it.  The  formula  de- 
mands 59.8  per  cent  Fe,  25.7  per  cent  O  and  14.5  per  cent  water,  but  the 
percentages  of  these  constituents  found  in  different  specimens  only 
approximately  correspond  to  these  figures.     Many  mineralogists  regard 


DESCRIPTIVE  MINERALOGY 


Fig.  89.— Limonite  Stalactites  in  Silverbow  Mine,  Butte,  Mont.     (After  W.  H.  Weed.} 


Fig.  90. — Botryoidal  Limonite. 


HYDROXIDES  185 

limonite  as  colloidal  goethite  (FeO-OH)  with  one  molecule  or  more  of 
H2O,  depending  upon  temperature.  The  principal  impurities  are  clay, 
sand,  phosphates,  silica,  manganese  compounds  and  organic  matter. 
The  great  variety  of  these  is  thought  to  be  due  to  the  fact  that  the 
limonite,  like  other  gels,  possesses  the  power  of  absorbing  compounds 
from  their  solution,  so  that  the  mineral  is  in  reality  a  mixture  of  col- 
loidal iron  hydroxide  and  various  compounds  which  differ  in  different 
occurrences. 

The  mineral  occurs  in  stalactites  (Fig.  89),  in  botiyoidal  forms  (Fig. 
90),  in  concretionary  and  clay-like  masses  and  often  as  pseudomorphs 
after  other  minerals  and  after  the  roots,  leaves  and  stems  of  trees. 

Limonite  is  brown  on  a  fresh  fracture,  though  the  surface  of  many, 
specimens  is  covered  with  a  black  coating  that  is  so  lustrous  as  to  appear 
varnished.  Its  streak  is  yellowish  brown.  Its  hardness  is  a  little  over 
5  and  its  density  about  3.7.  The  mineral  is  opaque  and  its  luster  is  dull, 
silky  or  almost  metallic  according  to  the  physical  conditions  of  the  spec- 
imen. Its  index  of  refraction  is  about  2.5.  It  is  a  nonconductor  of 
electricity. 

The  varieties  recognized  are:  compact,  the  stalactitic  and  other 
fibrous  forms;  ocherous,  the  brown  or  yellow  earthy,  impure  variety; 
bog  iron,  the  porous  variety  found  in  marshes,  pseudomorphing  leaves, 
etc.,  and  brown  clay  ironstone,  the  compact,  massive  or  nodular 
form. 

In  its  chemical  properties  limonite  resembles  goethite,  from  which  it 
can  be  distinguished  only  with  great  difficulty  except  when  the  latter  is 
in  crystals.  From  uncrystallized  varieties  of  goethite  it  can  usually  be 
distinguished  only  by  quantitative  analysis,  although  in  pure  specimens 
the  streaks  are  different. 

Occurrence  and  Origin. — Limonite  is  the  usual  result  of  the  decom- 
position of  other  iron-bearing  minerals.  Consequently,  it  is  often  found 
in  pseudomorphs.  In  almost  all  cases  where  large  beds  of  the  ore  occur 
the  material  has  been  deposited  from  ferriferous  water  rich  in  organic 
substances.  One  of  the  commonest  types  of  occurrence  is  "  gossan." 
In  the  production  of  this  type  of  ore,  those  portions  of  veins  carrying 
ferruginous  minerals  are  oxidized  under  the  influence  of  oxygen-bearing 
waters,  forming  a  layer  composed  largely  of  limonite  which  covers  the 
upper  portion  of  the  veins  and  hides  the  original  vein  matter.  Gossan 
ores  derived  from  chalcopyrite  and  pyrite  are  common  in  all  regions  in 
which  these  minerals  occur.  Another  type  of  limonitic  ore  comprises 
those  found  in  clays  derived  from  limestones  by  weathering.  In  such 
deposits  the  ore  occurs  as  nodules  and  in  pockets  in  the  clay.     Ores  of 


186  DESCRIPTIVE  MINERALOGY 

this  type  are  common  in  the  valleys  within  the  Appalachian  Moun- 
tains. Bog  iron  ores  occur  in  swamps  and  lakes  into  which  ferruginous 
solutions  drain.  The  iron  may  come  from  pyrite  or  iron  silicates  in  the 
drainage  basins  of  the  lakes  or  swamps.  When  carried  down  it  is  oxi- 
dized by  the  air  and  sinks  to  the  bottom. 

Localities. — The  mineral  occurs  abundantly  and  in  many  different 
localities.  The  most  important  American  occurrences  are  extensive 
beds  at  Salisbury  and  Kent,  Conn.;  at  many  points  in  New  Jersey, 
Pennsylvania,  Michigan,  Tennessee,  Alabama,  Ohio,  Virginia  and 
Georgia. 

Uses. — Although  containing  less  iron  than  hematite,  on  account  of 
its  cheapness,  and  the  ease  with  which  it  works  in  the  furnace,  brown 
hematite  is  an  important  ore  of  this  metal.  The  earthy  varieties  are 
used  as  cheap  paints. 

Production. — The  yield  of  the  United  States  "  brown  hematite  " 
mines  for  191 2  was  a  little  over  1,600,000  tons.  Of  this  amount  the 
largest  yields  were: 

Alabama .• .  749,242  tons 

Virginia 398,833  tons 

Tennessee 171,130  tons 

The  quantity  of  ocher  produced  in  the  United  States  during  the  same 
year  amounted  to  about  15,269  tons,  valued  at  $149,289.  Most  of  it 
came  from  Georgia.  In  addition,  8,020  tons  were  imported.  This 
had  a  value  of  $148,300. 

Bauxite  (Al20(OH)4) 

Bauxite,  or  beauxite,  like  limonite,  is  probably  a  colloid.  At  any 
rate  it  is  unknown  in  crystals.  Until  recently  it  possessed  but  little 
value.  It  is  now,  however,  of  considerable  importance  as  it  is  the  prin- 
cipal source  of  the  aluminium  on  the  market. 

The  mineral  is  apparently  an  hydroxide  of  aluminium  with  the  for- 
mula Al20(OH)4  or  A1203-2H20  in  which  26.1  per  cent  is  water  and 
73.9  per  cent  alumina  (AI2O3),  but  it  may  be  a  colloidal  mixture  of  the 
gibbsite  and  diaspore  (p.  190)  molecules,  or  of  various  hydroxides, 
since  its  analyses  vary  within  wide  limits.  A  sample  of  very  pure 
material  from  Georgia  gave  on  analysis: 


A1203 

Fe203 

S1O2 

Ti02 

H20 

62.46 

.81 

4-72 

23 

31.0* 

HYDROXIDES  187 

Bauxite  occurs  in  concretionary  grains  (Fig.  91),  in  earthy,  clay-like 
forms  and  massive,  usually  in  pockets  or  lenses  in  clay  resulting  from  the 
weathering  of  limestones  or  of  syenite.  It  is  white  when  pure,  but  as 
usually  found  is  yellow,  gray,  red  or  brown  in  color,  is  translucent  to 
opaque  and  has  a  colorless  or  very  light  streak.  Its  density  is  2.55 
and  its  hardness  anywhere  between  1  and*3.  Its  luster  is  dull.  It  is 
a  nonconductor  of  electricity. 

Before  the  blowpipe  bauxite  is  infusible.    In  the  closed  tube  it  yields 


Fig.  91.— Pisolitic  Bauxite,  from  near  Rock  Run,  Cherokee  Co.,  Ala. 

water  at  a  high  temperature.  Its  powder  when  intensely  heated  with  a 
few  drops  of  cobalt  nitrate  solution  turns  blue.  The  mineral  is  with 
difficulty  soluble  in  hydrochloric  acid. 

Occurrence  and  Origin. — Bauxite  in  some  cases  may  be  a  deposit  from 
hot  alkaline  waters,  but  in  Arkansas  it  is  a  residual  weathering  product 
of  the  igneous  rock,  syenite.  It  occurs  in  beds  associated  with  corundum, 
clay,  gibbsite  and  other  aluminium  minerals. 

Localities. — Large  deposits  of  the  ore  occur  at  Baux,  near  Aries, 
France;  near  Lake  Wochein,  in  Carniola;  in  Nassau;  at  Antrim,  Ire- 
land;  in  a  stretch  of  country  between  Jacksonville,  Fla.,  and  Carters- 


188  DESCRIPTIVE  MINERALOGY 

ville,  Ga.;  in  Saline  and  Pulaski  Counties,  Ark.;  in  Wilkinson  Co.,  Ga., 
and  near  Chattanooga,  Tenn. 

Preparation. — The  ore  is  mined  by  pick  and  shovel,  crushed  and 
washed.  It  is  then,  in  some  cases,  dried  and  broken  into  fine  particles. 
The  fine  dust  is  separated  from  the  coarser  material,  and  the  latter, 
which  comprises  most  of  the  ore,  is  heated  to  4000.  This  changes  the 
iron  compounds  to  magnetic  oxide  which  is  separated  electro-mag- 
netically.  The  concentrate  contains  about  86  per  cent  of  AI2O3.  This 
is  then  purified  and  dissolved  in  a  molten  flux,  in  some  cases  cryolite, 
and  is  subjected  to  electrolysis.  The  quantity  of  aluminium  made  in  the 
United  States  during  191 2  was  over  65,600,000  lb.,  valued  at  about 
$17,000,000.  The  value  of  the  aluminium  salts  produced  was  about 
$3,000,000. 

Uses. — Bauxite  (or  more  properly  the  mixture  of  bauxite  and  gibbs- 
ite)  is  practically  the  only  commercial  ore  of  aluminium  which,  on 
account  of  its  lightness  and  its  freedom  from  tarnish  on  exposure,  has 
become  a  very  popular  metal  for  use  in  various  directions.  It  is  em- 
ployed in  castings  where  light  weight  is  desired  and  in  the  manufacture 
of  ornaments  and  of  plates  for  interior  metallic  decorations.  It  is  also 
employed  in  the  steel  industry,  and,  in  the  form  of  wire,  for  the  trans- 
mission of  electricity.  The  mineral  is  also  used  in  the  manufacture  of 
aluminium  salts,  in  making  alundum  (artificial  corundum),  and  bauxite 
brick  for  lining  furnaces,  and  in  the  manufacture  of  paints  and  alloys. 

Production. — The  bauxite  mined  in  the  United  States  during  191 2 
amounted  to  about  159,865  tons  valued  at  $768,932,  the  greater  portion 
coming  from  Arkansas.  This  is  about  two-thirds  the  value  of  the  pro- 
duction of.  the  entire  world. 

Psilomelane 

Psilomelane  is  probably  a  mixture  of  colloidal  oxides  and  hydroxides 
of  manganese  in  various  proportions.  In  most  specimens  there  is  a 
notable  percentage  of  BaO  or  K2O  present,  and  in  others  small  quantities 
of  lithium  and  thallium.  The  barium  and  potassium  components  are 
thought  to  have  been  absorbed  from  their  solutions. 

The  substance  occurs  in  globular,  botryoidal,  stalactitic,  and  massive 
forms  exhibiting,  in  many  instances,  an  obscure  fibrous  structure.  Its 
color  is  black  or  brownish  black  and  its  streak  brownish  black  and 
glistening.    Its  hardness  is  5.5-6  and  specific  gravity  4.2. 

Psilomelane  is  infusible  before  the  blowpipe,  in  some  cases  coloring 
the  flame  green  (Ba)  and  in  others  violet  (K).     With  fluxes  it  reacts  for 


HYDROXIDES  189 

manganese.  In  the  closed  tube  it  yields  water.  It  is  soluble  in  HC1 
with  evolution  of  chlorine. 

It  is  distinguished  from  most  other  manganese  oxides  and  hydroxides 
by  its  greater  hardness. 

Occurrence. — Psilomelane  occurs  in  veins  associated  with  pyrolusite 
and  other  manganese  compounds,  as  nodules  in  clay  beds,  and  as  coatings 
on  many  manganiferous  minerals.  In  all  cases  it  is  probably  a  product 
of  weathering. 

Localities. — It  is  found  in  large  quantity  at  Elgersburg  in  Thuringia; 
at  Ilfeld,  Harz;  and  at  various  places  in  Saxony.  In  the  United  States 
it  occurs  with  pyrolusite  and  other  ores  of  manganese  at  Brandon,  Vt. ; 
in  the  James  River  Valley,  and  the  Blue  Ridge  region  of  Virginia;  in 
northeastern  Tennessee;  at  Cartersville,  Georgia;  at  Batesville,  Arkan- 
sas; and  in  a  stretch  of  country  about  forty  miles  southeast  of  San 
Francisco,  California.  At  many  of  these  points  it  has  been  mined  as  an 
ore  of  manganese. 

Wad 

Wad  is  a  soft,  earthy,  black  or  dark  brown  aggregate  of  manganese 
compounds  closely  related  to  psilomelane. 

It  occurs  in  globular,  botryoidal,  stalactitic,  flaky  and  porous 
masses,  which,  in  some  cases,  are  so  light  that  they  float  on  water.  It 
also  occurs  in  fairly  compact  layers  and  coats  the  surfaces  of  cracks, 
often  forming  branching  stains,  known  as  dendrites. 

Wad  contains  more  water  than  psilomelane,  of  which  it  appears 
often  to  be  a  decomposition  product.  More  frequently  it  results  from 
the  weathering  of  manganiferous  iron  carbonate.  It  is  particularly 
abundant  in  the  oxidized  portions  of  veins  containing  manganese  car- 
bonates and  silicates. 

Wad  is  easily  distinguished  from  all  other  soft  black  minerals,  except 
pyrolusite,  by  the  reaction  for  manganese,  and  from  all  other  manganese 
compounds,  except  pyrolusite,  by  its  softness.  From  pyrolusite  it  is 
distinguished  by  its  content  of  water. 

Localities. — It  occurs  in  most  of  the  localities  at  which  other  man- 
ganese compounds  are  found. 

DIASPORE  GROUP 

The  diaspore  group  comprises  the  hydroxides  of  aluminium,  iron 
and  manganese,  possessing  the  general  formula  R"'0(OH).  They  are 
regarded  as  hydroxides  in  which  one  of  the  hydrogens  in  H2O  is  replaced 
by  the  group  R'"0,  thus:  H — O — H,  water,  AlO — O — H,  diaspore.  These 


190 


DESCRIPTIVE  MINERALOGY 


three  compounds  from  a  chemical  viewpoint,  may  be  looked  upon  as  the 
acids  whose  salts  comprise  the  spinel  group  of  minerals,  which  includes 
among  others  the  three  important  ore  minerals  magnetite,  chromite  and 
franklinite.  Of  the  three  members  of  the  diaspore  group  the  manganese 
and  iron  compounds  are  valuable  ores.  All  are  orthorhombic,  in  the 
rhombic  bipyramidal  class. 

Diaspore  (AIO(OH)) 

Diaspore  is  found  in  colorless  or  light  colored  crystals,  in  foliated 
masses  and  in  stalactitic  forms. 

Its  composition  is  theoretically  85  per  cent  AI2O3  and  15  per  cent 


tf^H5v 


m 


v? 


V 


Fig.  92. — Diaspore  Crystals.  00  P  06  ,  010  (6);  *>  P3,  130  (z);  00  P,  no  (m);  00  P2, 
210  (h);  P£,  on  (e),  P2,  212  (s);  00  P2,  120  (/);  °o  P$,  150  (n);  |P|, 
232  (tf). 

H2O,  though  analyses  show  it  to  contain,  in  addition,  usually,  some  iron 
and  silicon.    A  specimen  from  Pennsylvania  yielded: 


AI2O3 
80.95 


H20 

14.84 


Fe203 
3.12 


Si02      Total 
1.53     100.44 


Other  specimens  approach  the  theoretical  composition  very  closely. 

In  crystallization  the  mineral  is  orthorhombic  (rhombic  bipyramidal 
class),  with  a  :  b  :  ^=.9372  :  1  :  .6039.  The  crystals  are  usually  pris- 
matic, though  often  tabular  parallel  to  00  Poo  (010).  The  principal 
planes  observed  on  them  are  00  Poo  (010),  a  series  of  prisms  as 
ooP(no),  ooP2(2io),  °oP3(i3o),  the  dome  Poo  (on)  and  several 
pyramids  (Fig.  92).  The  planes  of  the  prismatic  zone  are  often  ver- 
tically striated.    The  angle  iioAi^o  =  86°  17'. 

The  cleavage  of  diaspore  is  very  distinct  parallel  to  the  brachy- 
pinacoid.  Its  fracture  is  conchoidal  and  the  mineral  is  very  brittle. 
Its  hardness  is  about  6.5  and  density  3.4.  The  luster  of  the  mineral  is 
vitreous,  except  on  the  cleavage  surface,  where  it  is  pearly.    Its  color 


HYDROXIDES  191 

varies  widely,  though  the  tint  is  always  light  and  the  streak  colorless. 
The  predominant  shades  are  bluish  white,  grayish  white,  yellowish  or 
greenish  white.  The  mineral  is  transparent  or  translucent.  It  is  a 
nonconductor  of  electricity.  Its  refractive  indices  for  yellow  light  are: 
a=.i702, 18=1.722,  7  =  1.750. 

In  the  closed  tube  diaspore  decrepitates  and  gives  off  water  at  a  high 
temperature.  It  is  infusible  and  insoluble  in  acids.  When  moistened 
with  a  solution  of  cobalt  nitrate  and  heated  it  turns  blue,  as  do  all  other 
colorless  aluminium  compounds. 

In  appearance,  diaspore  closely  resembles  bruciie  (Mg(OH)2),  from 
which  it  may  be  distinguished  by  its  greater  hardness  and  its  aluminium 
reaction  with  cobalt  nitrate. 

Synthesis. — Crystal  plates  of  diaspore  have  been  made  by  heating  at 
a  temperature  of  less  than  5000,  an  excess  of  amorphous  AI2O3  in  sodium 
hydroxide,  enclosed  in  a  steel  tube.  At  a  higher  temperature  corundum 
resulted. 

Occurrence. — Diaspore  occurs  as  crystals  implanted  on  corundum 
and  other  minerals,  and  on  the  walls  of  rocks  in  which  corundum  is 
found.  It  is  probably  in  most  cases  a  decomposition  product  of  other 
aluminium  compounds. 

Localities. — In  Ekaterinburg,  Russia,  it  is  associated  with  emery. 
At  Schemnitz,  Hungary,  it  occurs  in  veins.  It  is  found  also  in  the 
Canton  of  Tessin,  in  Switzerland,  at  various  places  in  Asia  Minor,  and 
on  the  emery-bearing  islands  of  the  Grecian  Archipelago.  In  the 
United  States  it  is  associated  with  corundum,  at  Newlin,  Chester  Co., 
Penn.,  with  emery  at  Chester,  Mass.,  at  the  Culsagee  corundum  mine, 
near  Franklin,  N.  C,  and  at  other  corundum  mines  in  the  same  State. 

Manganite  (MnO(OH)) 

Manganite  usually  occurs  in  groups  of  black  columnar  or  prismatic 
crystals  and  in  stalactites. 

The  formula  MnO(OH)  requires  27.3  per  cent  O,  62.4  per  cent  Mn 
and  10.3  per  cent  water,  or  89.7  per  cent  MnO  and  10.3  per  cent  water. 
In  addition  to  these  constituents,  the  mineral  commonly  contains  also 
some  iron,  magnesium,  calcium  and  often  traces  of  other  metals.  An 
analysis  of  a  specimen  from  Langban,  in  Sweden,  yielded: 

Mn203  Fe203         MgO       CaO        H20        Total 

88.51  .23  1. 51        .62         9.80        100.67 

The  orthorhombic  crystals  of  the  mineral  have  an  axial  ratio  a  :  b  :  c 
=  .8441  :  1  :  .5448.    The  crystals  are  nearly  all  columnar  with  a  series 


192  DESCRIPTIVE  MINERALOGY 

of  prisms,  among  which  are  w  P4(4io)  and  °o  P(i  10),  and  the  two  lateral 
pinacoids  ooP  06(010)  and  8  P  60  (100)  terminated  by  oP(ooi)  or  by 
the  domes  PS  (on),  P*  (101),  and  pyramids  (Figs.  93  and  04).  Cru- 
ciform and  contact  twins,  with  the  twinning  plane  P  06  (on),  are  not 
uncommon  (Fig.  95).  The  prismatic  surfaces  are 
I  vertically  striated  and   the  crystals  are  often  in 

bundles.    The  angle  1 10  A  tio—  8o°  20'. 

Cleavage  is  well  defined  parallel  to  00  P  06  (010) 
and  less  perfectly  developed  parallel  to   qoP(iio). 
The  fracture  is  uneven.    The  luster  of  the  mineral 
is  brilliant,  almost  metallic.     Its  color  is  iron-black 
and  its  streak  reddish  brown  or  nearly  black.    It 
Fig.  93.  —  Manganite    -K  usually  opaque  but  in  very  thin  splinters  it  is 
Crystal  "1th  ™p'    sometimes  brown  by  transmitted  light.     Its  hard- 
and  p"m  "ioH")       ness  **  4  an<*  density  about  4-3-    The  mineral  is 
a  nonconductor  of  electricity. 
Manganite  yields  water  in  the  closed  tube  and  colors  the  borax  bead 
amethyst  in  the  oxidizing  flame  of  the  blowpipe.    In  the  reducing  flame, 
upon  long-continued  heating,  this  color  disappears.    The  mineral  dis- 
solves in  hydrochloric  acid  with  the  evolution  of  chlorine.    It  is  dis- 


FlO.  94- — Group  of  Prismatic  Manganite  Crystals  from  Ilfeld,  Harz. 

tinguished  from  other  manganese  minerals  by  its  hardness  and  crystal- 
lization. 

By  loss  of  water  manganite  passes  readily  into  pyrolusite  (MnOs). 
It  also  readily  alters  into  other  manganese  compounds. 

Synthesis. — Upon  heating  for  six  months  a  mixture  of  manganese 
chloride  and  clcium  caarbonate  fine  crystals  like  those  of  manganite 


HYDROXIDES  193 

have  been  obtained.  Their  composition,  however,  was  that  of  haus- 
mannite,  indicating  that  while  manganite  was  produced,  it  was  changed 
to  hausmannite  during  the  process. 

Occurrence,  Localities  and  Origin, — Man- 
ganite occurs  in  veins  in  old  volcanic  rocks, 
and  also  in  limestone.  It  is  found  at  Ilfeld 
in  the  Harz;  at  Ilmenau  in  Thuringia,  and 
at  Langban  in  Sweden,  in  handsome  crys- 
tals. In  the  United  States  it  occurs  at  the 
Jackson  and  the  Lucie  iron  mines,  Negaunee, 

Mich.,  and    in   Douglas    Co.,  Colo.     It  is    ^  ,,         .     ^ 

,         ,        ,     A  .  .  .      XT         Fig.  95. — Manganite  Crystal 

also  abundant   at   various  places  in  New       Twhmed  about  p5(oxi)b 

Brunswick  and  Nova  Scotia.  In  all  cases  The  forms  are  <»P.  no(«); 
it  is  a  residual  product  of  the  weathering  of  ooPXiaoCOandPa^iaQ;). 
manganese  compounds. 

Uses. — Manganite  is  used  in  the  production  of  manganese  compounds. 
As  mined  it  is  usually  mixed  with  pyrolusite,  this  being  the  most  im- 
portant portion  of  the  mixture. 

Goethite  (FeO(OH)) 

This  mineral,  though  occasionally  found  in  blackish  brown  crystals, 
usually  occurs  in  radiated  globular  and  botryoidal  masses.  Analyses 
of  specimens  from  Maryland,  and  from  Lostwithiel,  in  Cornwall,  gave: 

FC2O3         Mn203 

Maryland 86.32 

Lostwithiel 89-55  •  J6 

The  formula  FeO(OH)  demands  89.9  per  cent  Fe203  and  10. 1  per  cent 
H2O  or  62.9  per  cent  Fe,  27.0  per  cent  O  and  10. 1  per  cent  H2O. 

Like  diaspore  and  manganite,  goethite  is  orthorhombic,  its  axial 
ratio  being  a  :  b  :  ^  =  .9185  :  1  :  .6068.  Its  crystals  are  prismatic  or 
acicular  with  the  prisms  plainly  striated  vertically.  The  forms  observed 
are  commonly  00  P  66  (010),  ooP2~(2io),ooP(no),P66  (on)  andP(ni). 
The  angle  no  A  1 10=85°  8'. 

The  cleavage  of  goethite  is  perfect  parallel  to  00  P  06  (010)  and  its 
fracture  uneven.  Its  hardness  is  5  and  density  about  4.4.  Its  color  is 
usually  yellowish,  reddish  or  blackish  brown  and  its  luster  almost 
metallic.  In  thin  splinters  it  is  often  translucent  with  a  blood-red  color 
and  a  refractive  index  of  about  2.5.  Its  streak  is  brownish  yellow.  It 
is  an  electric  nonconductor. 


H20 

Si02 

Total 

10.80 

2.88 

100.00 

10.07 

.28 

100.06 

194  DESCRIPTIVE  MINERALOGY 

The  chemical  reactions  of  the  mineral  are  about  the  same  as  those  of 

0 

hematite,  except  that  it  yields  water  when  heated  in  the  closed  tube. 
By  this  reaction  it  is  easily  distinguished  from  the  fibrous  varieties  of 
hematite,  as  it  is  also  by  its  streak. 

Synthesis. — Needles  of  goethite  are  produced  by  heating  freshly 
precipitated  iron  hydroxide  for  a  long  time  at  ioo°. 

Occurrence  and  Localities. — Goethite  is  usually  associated  with  other 
ores  of  iron,  especially  in  the  upper  portion  of  veins,  where  it  is  produced 
by  weathering.  It  is  found  near  Siegen  in  Nassau;  near  Bristol  and 
Clifton,  England,  and  in  large,  fine  crystals  at  Lostwithiel  and  other 
places  in  Cornwall. 

In  the  United  States  it  occurs  in  small  quantity  at  the  Jackson  and 
the  Lucie  hematite  mines  in  Negaunee,  Mich.;  at  Salisbury,  Conn.; 
at  Easton,  Penn.,  and  at  many  other  places. 

Uses. — Goethite  is  used  as  an  ore  of  iron,  but  in  the  trade  it  is  classed 
with  limonite  as  brown  hematite. 


CHAPTER  DC 
THE  ALUMINATES,    FERRITES,    CHROMITES   AND   MANGANITES 

Most  of  these  compounds  are  salts  of  the  comparatively  uncommon 
acids  HAIO2,  HFeCfe  and  HC1O2,  which  may  be  regarded  as  metaacids 
derived  from  the  corresponding  normal  acids  by  the  abstraction  of  water, 
thus:  H3AIO3— H20  =  HA102.  There  are  only  a  few  minerals  belong- 
ing to  the  group  but  they  are  important.  One,  magnetite,  is  an  ore  of 
iron;  another,  chromite,  is  the  principal  ore  of  chromium  and  two  others 
are  utilized  as  gems.  Most  of  them  are  included  in  the  mineral  group 
known  as  the  spinels.     (Compare  p.  189.) 

That  there  is  a  manganese  acid  corresponding  to  the  metaacids  of  Al, 
Fe  and  Cr  is  indicated  by  the  fact  that  in  some  of  the  spinels  manganese 
replaces  some  of  the  ferric  iron,  as,  for  example,  in  franklinite.  This 
suggests  that  this  mineral  is  an  isomorphous  mixture  of  a  metaferrite 
and  a  salt  of  the  corresponding  manganese  acid  (HMnCfe).  This  may 
be  regarded  as  derived  from  the  hydroxide,  Mn(OH)3,  by  abstraction 
of  H2O,  thus:  HsMn03— H20=HMn02.  But  there  are  other  man- 
ganous  acids.  Normal  manganous  acid  is  Mn(OH)4,  or  HUMnC^.  If 
from  this  one  molecule  of  water  be  abstracted,  there  remains  H2Mn03, 
the  metamanganous  acid.  The  manganous  salt  of  the  normal  acid, 
Mn2Mn04,  occurs  as  the  mineral,  hausmannite,  and  the  corresponding 
salt  of  the  metaacid,  MnMnC>3,  as  the  mineral,  braunite. 

SPINEL  GROUP 

The  spinels  are  a  group  of  isomorphous  compounds  that  may  be 
regarded  as  salts  of  the  acids  AIO(OH),  MnO(OH),  CrO(OH)  and 
FeO(OH),   in  which   the   hydrogen   is  replaced  by  Mg,  Fe  and  Cr. 

Al— O— Ov 

Thus,  spinel,  Mg- AI2O4  may  be  regarded  as  ||  /Mg;   magnetite, 

Al— O— (K 
Fe— O— (X  Cr— O— Ov 

Fe304,  as  ||  /Fe;   cl.romite,  FeCr204,  as   ||  /Fe;  and 

Fe-O— (X  Cr— O-CX 

(Fe  •  Mn)— 0— <X 
frank1  inite,  as    |      |  y(Zn-Mn-Fe).   Chemical  compounds  of 

(Fe  Mn)— O— Cr 

195 


196 


DESCRIPTIVE  MINERALOGY 


this  general  type  are  fairly  numerous,  but  only  a  few  occur  as  minerals. 

The  most  important  are  the  three  important  ores  mentioned  above; 

spinel  is  of  some  value  as  a  gem  stone. 

The  spinels  crystallize  in  the  holohedral  divi- 
sion of  the  isometric  system  (hexoctahedral  class), 
in  well  defined  crystals  that  are  usually  combina- 
tions of  O(iii)  and  ooO(no),  with  the  addition  on 
some  of  00O00  (ioo),  303(311),  202(211),  5OK531), 
etc.  Contact  twins  are  so  common  with  O  the 
twinning  plane,  that  this  type  of  twinning  is  often 
referred  to  as  the  spinel  twinning  (Fig.  96). 


Fig.  96. 
Spinel  Twin. 


The  complete  list  of  the  known  spinels  is  as  follows: 


Spinel 

Ceylonite  (pleonaste) 

Chlorspinel 

Picotite 

Hercynite 

Gahnite 

Dysluite 

Kreittonite 

Magnetite 

Magnesioferrite 

Franklinite 

Jacobsite 

Chromite 


Mg(A102)2 
(Mg.Fe)(A102)2 

Mg((Al-Fe)02)2 

(Mg-Fe)((Al-Fe-Cr)02)2 

Fe(A102)2 

Zn(A102)2 

(Zn-Fe-Mn)((Al-Fe)02)2 

(Zn-Fe-Mg)  ((Al-Fe)02)2 

Fe(Fe02)2 

Mg(Fe02)2 

(Fe  •  Zn.Mn)  ((Fe  •  Mn)02)2 

(Mn-Mg)((Fe-Mn)02)2 

(Fe-Mg)(Cr-Fe)02)2 


Spinel  (Mg(A102)2) 

Ordinary  spinel  is  the  magnesian  aluminate,  which,  when  pure,  con- 
tains  28.3   per  cent  MgO  and   71.7   per  cent 
Al203.    Usually,  however,    there  are    present 
admixtures   of   the   other   isomorphs   so   that 
analyses  often  indicate  Fe,  Al  and  Cr. 

The  mineral  usually  occurs  in  isolated 
simple  crystals,  rarely  in  groups.  The  forms 
observed  on  them  are  O(iu),  ooO(no)  and 
303(311),  and  rarely  00  O  00  (100)  (Fig.  97). 

The  pure  magnesium  spinel  is  colorless  or  FlG*.  97-— Spinel    Crystal 

some  shade  of  pink  or  red,  brown  or  blue,  and      T?N      \ x"  w>        ' 

v  '  '  \d)  and  303,  311  (m). 

is  usually  transparent  or  translucent,  though 

opaque  varieties  are  not  rare.    Its  streak  is  white.    It  possesses  a  glassy 


ALUMINATES,  FERRITES,  ETC.  197 

luster,  and  a  conchoidal  fracture,  but  no  distinct  cleavage.  Its  hard- 
ness is  8  and  its  density  3.5-3.6.  Its  refractive  indices  vary  with  the 
color:  n  for  yellow  light  is  1.7150  for  red  spinel  and  1.7201  for  the  blue 
variety. 

The  mineral  is  infusible  before  the  blowpipe  and  is  unattacked  by 
acids.    It  yields  the  test  for  magnesia  with  cobalt  solution. 

Spinel  is  easily  distinguished  from  most  other  minerals  by  its  crys- 
tallization and  hardness.  It  is  distinguished  from  pale-colored  garnet 
by  its  blowpipe  reactions,  especially  its  infusibility,  and  its  failure  to 
respond  to  the  test  for  Si02. 

The  best  known  varieties  are: 

Precious  spinel,  which  is  the  pure  magnesian  aluminate.  It  is  trans- 
parent and  colorless  or  some  light  shade  of  red,  blue  or  green.  The 
bright  red  variety  is  known  as  ruby  spinel  and  is  used  as  a  gem.  Its 
color  is  believed  to  be  due  to  the  presence  of  chromium  oxide.  It  is 
easily  distinguished  from  genuine  ruby  by  the  fact  that  it  is  not  doubly 
refracting  and  not  pleochroic. 

The  best  ruby  spinels  come  from  Ceylon,  where  they  occur  loose  in 
sand  associated  with  zircon,  sapphire,  garnet,  etc. 

Common  spinel  differs  from  precious  spinel  in  that  it  is  translucent. 
It  usually  contains  traces  of  iron  and  alumina. 

Both  these  varieties  of  spinel  occur  in  metamorphosed  limestones 
and  crystalline  schists. 

Syntheses. — The  spinels  have  been  made  by  heating  a  mixture  of 
AI2O3  and  MgO  with  boracic  acid  until  fusion  ensues;  and  by  heating 
Mg(OH)2  with  AICI3  in  the  presence  of  water  vapor. 

Origin. — Spinel  has  been  described  as  an  alteration  product  of  corun- 
dum and  garnet.  It  is  also  a  primary  component  of  igneous  rocks  and 
a  product  of  metamorphism  in  rocks  rich  in  magnesium. 

Uses. — Only  the  transparent  ruby  spinels  have  found  a  use.  These 
are  employed  as  gems. 

Ceylonite,  or  pleonaste,  is  a  spinel  in  which  a  portion  of  the  Mg 
has  been  replaced  by  Fe,  i.e.,  is  an  isomorphous  mixture  of  the  magne- 
sian and  iron  aluminates;  thus  ((Mg-Fe)(A102)2).  It  is  usually  black 
or  green  and  translucent,  and  has  a  brownish  or  dark  greenish  streak 
and  a  density =3.5-3.6. 

The  analysis  of  a  sample  separated  from  an  igneous  rock  in  Madison 
Co.,  Mont.,  gave, 

AI2O3        FeO      MgO     Cr203     Fe203     MnO     CaO     SiCfe    Total 
62.09      17.56     15.61       2.62        2.10        tr  .16        .55     100.69 


198  DESCRIPTIVE  MINERALOGY 

Ceylonite  occurs  in  igneous  rocks  in  the  Lake  Laach  region, 
Germany,  and  in  the  Piedmont  district,  Italy  and  elsewhere;  in  meta- 
morphosed limestones  at  Warwick  and  Amity,  N.  Y. ;  in  the  limestone 
blocks  enclosed  in  the  lava  of  Vesuvius;  and  in  dolomite  metamor- 
phosed by  contact  action  at  Monzoni,  Tyrol. 

Picotite,  or  chrome  spinel,  is  a  variety  intermediate  between  spinel 
proper  and  chromite.  Its  composition  may  be  represented  by  the 
formula  (Mg-Fe)((Al-Fe- 0)02)2.  It  occurs  only  in  small  crystals  in 
basic  igneous  rocks  and  in  a  few  crystalline  schists.    Density =4.1. 

Magnetite  (Fe(Fe02)2) 

Magnetite,  the  ferrous  ferrite,  the  empirical  formula  of  which  is 
FesOi,  is  a  heavy,  black,  magnetic  mineral  which  is  utilized  as  one  of 
the  ores  of  iron. 

The  pure  mineral  consists  of  72.4  per  cent  Fe  and  27.6  per  cent  O. 
Most  specimens,  however,  contain  also  some  Mg  and  many  contain  small 
quantities  of  Mn  or  Ti.  A  selected  sample  of  magnetite  from  the  Eliza- 
beth Mine,  Mt.  Hope,  New  Jersey,  analyzed  as  follows: 

Fe203       FeO       Si02 
65 . 26      30 . 20      1 . 38 

Magnetite  occurs  in  crystals  that  are  usually  octahedrons  or  dodeca- 
hedrons, or  combinations  of  the  two.  Other  forms  are  rare.  Twins 
are  common.  The  mineral  occurs  also  as  sand 
and  in  granular  and  structureless  masses.  When 
the  dodecahedron  is  present,  its  faces  are  fre- 
quently striated  parallel  to  the  edge  between 
ooO(no)  and  O(in)  (Fig.  98). 

Magnetite  is  black  and  opaque  and  its  streak  Fig.  98.  — Magnetite 
is  black.  It  has  an  uneven  or  a  conchoidal  frac-  Crystal,  with  » o 
ture,  but  no  distinct  cleavage.    Its  hardness  is      ("<>)  and   O  (in), 

5.5-6  and  density  4.9-5.2.    It  is  strongly  attracted      p^rljlef  J  Edge'  1 10 
by  a  magnet  and  in  many  instances  it  exhibits      aiMi  lllm 
polar  magnetism. 

The  mineral  is  infusible  before  the  blowpipe.  Its  powder  dissolves 
slowly  in  HC1,  and  the  solution  reacts  for  ferrous  and  ferric  iron. 

Magnetite  is  easily  recognized  by  its  color,  magnetism,  and  hardness. 

The  mineral  weathers  to  limonite  and  hematite  and  occasionally  to 
the  carbonate,  siderite. 


Ti02 

AI2O3 

MgO 

CaO 

Other 

Total 

1.09 

•55 

.10 

.68 

•73 

99-99 

ALUMINATES,  FERRITES,  ETC.  199 

Syntheses. — Crystals  have  been  made  by  cooling  iron-bearing  silicate 
solutions;  treating  heated  ferric  hydroxide  with  HC1;  and  by  fusing 
iron  oxide  and  borax  with  a  reducing  flame. 

Occurrence  end  Origin. — The  mineral  occurs  as  a  constituent  of 
many  igneous  rocks  and  crystalline  schists,  and  in  lenses  embedded  in 
rocks  of  many  kinds.  It  also  constitutes  veins  cuttinjg  these  rocks 
and  as  irregular  masses  produced  by  the  dehydration  and  deoxidation 
of  hematite  and  limonite  under  the  influence  of  metamorphic  processes. 
It  occurs  also  as  little  grains  among  the  decomposition  products  of 
iron-bearing  silicates,  such  as  olivine  and  hornblende. 

The  larger  masses  are  either  segregations  from  igneous  magmas  or 
deposits  from  hot  solutions  and  gases  emanating  from  them. 

Localities. — The  localities  at  which  magnetite  has  been  found  are  so 
numerous  that  only  those  of  the  greatest  economic  importance  may  be 
mentioned  here.  In  Norway  and  Sweden  great  segregated  deposits  are 
worked  as  the  principal  sources  of  iron  in  these  countries.  In  the 
United  States  large  lenses  occur  in  the  limestones  and  siliceous  crys- 
talline schists  in  the  Adirondacks,  New  York,  and  in  the  schists  and 
granitic  rocks  of  the  Highlands  in  New  Jersey.  Great  bodies  are  mined 
also  at  Cornwall,  and  smaller  bodies  at  Cranberry,  and  in  the  Far 
West. 

Extraction. — The  magnetite  is  separated  from  the  rock  with  which  it 
occurs  by  crushing  and  exposing  to  the  action  of  an  electro-magnet. 

Production. — The  total  amount  of  the  mineral  mined  in  the  United 
States  during  191 2  was  2,179,500  tons,  of  which  1,110,345  tons  came 
from  New  York,  476,153  tons  from  Pennsylvania,  and  364,673  tons 
from  New  Jersey. 

Franklinite  ((Fe-Zn-Mn)((Fe-Mn)02)2) 

Franklinite  resembles  magnetite  in  its  general  appearance.  It  is  an 
ore  of  manganese  and  zinc. 

It  differs  from  magnetite  in  containing  Mn  in  place  of  some  of  the 
ferric  iron  in  this  mineral  and  Mn  and  Zn  in  place  of  some  of  its  ferrous 
iron.  Since  it  is  an  isomorphous  mixture  of  the  iron,  zinc  and  manganese 
salts  of  the  iron  and  manganese  acids  of  the  general  formula  R'"0(OH), 
its  composition  varies  within  wide  limits.  The  franklinite  from  Mine 
Hill,  N.  J.,  contains  from  39  per  cent  to  47  per  cent  Fe,  10  per  cent  to 
19  per  cent  Mn  and  6  per  cent  to  18  per  cent  Zn.  A  specimen  from 
Franklin  Furnace,  N.  J.,  contained, 

Fe203       MnO         ZnO       MgO        CaO        SiCfe         H20      Total 
66.58       9.96         20.77        .34         .43  .72  .71        99.51 


200  DESCRIPTIVE  MINERALOGY 

Its  crystals  are  usually  octahedrons,  sometimes  modified  by  the  do- 
decahedron and  occasionally  by  other  forms.  The  mineral  occurs  also 
in  rounded  grains,  in  granular  and  in  structureless  masses. 

It  is  black  and  lustrous  and  has  a  dark  brown  streak.  Its  fracture 
and  cleavage  are  the  same  as  for  magnetite.  It  is  only  very  slightly 
magnetic.    It  has  a  hardness  of  6  and  a  density  of  5.15. 

The  mineral  is  infusible  before  the  blowpipe.  When  heated  on 
charcoal  it  becomes  magnetic.  When  fused  with  Na2CC>3  in  the  oxidizing 
flame  it  gives  the  bluish  green  bead  characteristic  of  manganese.  Its 
fine  powder  mixed  with  Na2COa  and  heated  on  charcoal  yields  the  white 
coating  of  zinc  oxide  which  turns  green  when  moistened  with  Co(N03)2 
solution  and  again  heated. 

Franklinite  is  distinguished  from  most  minerals  by  its  color  and  crys- 
tallization and  from  magnetite  and  clromite  by  its  brown  streak  and 
by  its  reactions  for  Mn  and  Zn.  It  is  also  characterized  by  its  associa- 
tion with  red  zincite  and  green  or  pink  willemite  (p.  306). 

Synthesis. — Crystals  of  franklinite  have  been  made  by  heating  a 
mixture  of  FeCk,  ZnCk  and  CaO  (lime). 

Occurrence  and  Origin. — Franklinite  occurs  at  only  a  few  places.  Its 
most  noted  localities  are  Franklin  Furnace  and  Sterling  Hill,  N.  J.,  where 
it  is  associated  in  a  white  crystalline  limestone  with  zincite,  willemite 
and  other  zinc  and  manganese  compounds.  The  deposit  is  supposed 
to  have  been  produced  by  the  replacement  of  the  limestone  through  the 
action  of  magmatic  waters  and  vapors. 

Uses. — The  mineral  is  utilized  as  an  ore  of  manganese  and  zinc. 
The  ore  as  mined  is  crushed  and  separated  into  parts,  one  of  which 
consists  largely  of  franklinite.  This  is  roasted  with  coal,  when  the  zinc 
is  driven  off  as  zinc  oxide.  The  residue  is  smelted  in  a  furnace  producing 
spiegeleisen,  which  is  an  alloy  of  iron  and  manganese  used  in  the  man- 
ufacture of  certain  grades  of  steel. 

Production. — The  quantity  of  this  residuum  produced  in  191 2  was 
104,670  tons,  valued  at  $314,010. 

Chromite  (Fe(Cr02)2) 

Chromite,  or  chrome-iron,  is  the  principal  ore  of  chromium.  It 
resembles  magnetite  and  franklinite  in  appearance.  It  occurs  in  iso- 
lated crystals,  in  granular  aggregates,  and  in  structureless  masses. 

Chemically,  it  is  a  ferrous  salt  of  metachromous  acid,  of  the  theoret- 
ical composition  CT2O3  =  68  per  cent  and  FeO =32  per  cent,  but  it  usually 
contains  also  small  quantities  of  AI2O3,  CaO  and  MgO.    An  analysis  of 


ALUMINATES,   FERRITES,   ETC.  201 

a  specimen  from  Chorro  Creek,  California,  after  making  corrections  for 
the  presence  of  some  serpentine,  yielded: 


Cr203 

AI2O3 

Fe203 

FeO 

MgO 

MnO 

Total 

56.96 

12.32 

381 

12.73 

14.02 

.16 

100.00 

Its  crystals  are  usually  simple  octahedrons,  but  they  are  not  as 
common  as  those  of  the  other  spinels. 

Its  color  is  brownish  black  and  its  streak  brown.  It  has  a  conchoidal 
or  uneven  fracture  and  no  distinct  cleavage.  It  is  usually  nonmag- 
netic, but  some  specimens  show  slight  magnetism  because  of  the  ad- 
mixture of  the  isomorphous  magnetite  molecule.  Its  hardness  is  5.5 
and  its  density  4.5  to  4.8. 

The  mineral  is  infusible  before  the  blowpipe.  It  gives  the  chromium 
reaction  with  the  beads.  If  its  powder  is  fused  with  niter  and  the  fusion 
treated  with  water,  a  yellow  solution  of  K.2Cr04  results.  When  fused 
with  Na2COa  on  charcoal  it  yields  a  magnetic  residue. 

Chromite  is  easily  distinguished  from  all  other  minerals  but  pico- 
tite  by  its  crystallization  and  its  reaction  for  chromium.  It  is  distin- 
guished from  picotite  by  its  inferior  hardness  and  its  higher  specific 
gravity. 

Synthesis. — Crystals  have  been  made  by  fusing  the  proper  constit- 
uents with  boric  acid  and  after  fusion  distilling  off  the  boric  acid. 

Occurrence  and  Origin. — Chromite  occurs  principally  in  olivine  rocks 
and  in  their  alteration  product — serpentine.  The  mineral  is  found  not 
only  as  crystals  embedded  in  the  rock  mass,  but  also  as  nodules  in  it 
and  as  veins  traversing  it.  It  is  probably  in  all  cases  a  segregation  from 
the  magma  producing  the  rock.  In  a  few  places  the  mineral  occurs 
in  the  form  of  sand  on  beaches. 

Localities. — It  is  widely  spread  through  serpentine  rocks  at  many 
places,  notably  in  Brussa,  Asia  Minor;  at  Banat  and  elsewhere  in 
Norway;  at  Solnkive,  in  Rhodesia;  in  the  northern  portion  of  New 
Caledonia;  at  various  points  in  Macedonia;  in  the  Urals,  Russia;  in 
Beluchistan  and  Mysore,  India. 

In  the  United  States  the  mineral  is  known  at  several  points  in  a  belt 
of  serpentine  on  the  east  side  of  the  Appalachian  Mountains,  and  at 
many  points  in  the  Rocky  Mountains,  the  Sierra  Nevada  and  the  Coast 
Ranges.  It  has  been  mined  at  Bare  Hills,  Maryland;  in  Siskiyou, 
Tehama  and  Shasta  Counties,  Colorado;  in  Converse  County,  Wyoming; 
and  in  Chester  and  Delaware  Counties,  Pennsylvania;  and;  in  1914, 
some  was  washed  from  chrome  sand  at  Baltimore,  Maryland. 


202  DESCRIPTIVE  MINERALOGY 

Metallurgy. — The  mineral  is  mined  by  the  usual  methods  and  con- 
centrated, or,  if  in  large  fragments,  is  crushed.  It  is  then  fused  with 
certain  oxidizing  chemicals  and  the  soluble  chromates  are  produced. 
Or  the  ore  is  reduced  with  carbon  yielding  an  alloy  with  iron.  The 
metal  is  produced  by  reduction  of  its  oxide  by  metallic  aluminium  or  by 
electrolysis  of  its  salts. 

Uses. — Chromite  is  the  sole  source  of  the  metal  chromium,  which  is 
the  chrome-iron  alloy  employed  in  the  manufacture  of  special  grades 
of  steel.  Chromium  salts  are  used  in  tanning  and  as  pigments.  The 
crude  ore,  mixed  with  coal-tar,  kaolin,  bauxite,  or  some  other  ingredient, 
is  molded  into  bricks  and  burned,  after  which  the  bricks  are  used  as 
linings  in  metallurgical  furnaces.  These  bricks  stand  rapid  changes  of 
temperature  and  are  not  attacked  by  molten  metals. 

Production. — The  annual  production  of  chromite  in  the  world  is 
now  about  100,000  tons,  of  which  Rhodesia  produces  about  J,  New 
Caledonia  about  J  and  Russia  and  Turkey  about  fc  each.  The  produc- 
tion of  the  United  States  in  191 2  was  201  tons,  valued  at  $2,753.  All 
came  from  California.  The  imports  in  the  same  year  were  53,929  tons, 
worth  $499,818. 


Chrysoberyl  (BeAl204) 

Chrysoberyl  is  a  beryllium  aluminate,  the  composition  of  which  is 
analogous  to  that  of  the  spinels.  It  may  be  written  Be02(A10)2.  Al- 
though theoretically  it  should  contain  19.8  per  cent  BeO  and  80.2  per 
cent  AI2O3,  analyses  of  nearly  all  specimens  show  the  presence  also  of 
iron  and  magnesium. 

The  mineral  differs  from  spinel  in  crystallizing  in  the  orthorhombic 
system  (bipyramidal  class).  Its  axial  ratio  is  .4707  :  1  :  .5823.  The 
principal  forms  observed  on  crystals  are:  P(in),  00  P  66  ( 1 00) , 
00  P  66  (010),  P  06  (on),  00  P2(i2o)  and  2P2(i2i)  (Fig. 99).  The  crystals 
are  often  twins  (Fig.  100),  trillings  or  sixlings,  with  3Po6(o3i)  the 
twinning  plane,  forming  pseudohexagonal  groupings  (Fig.  101).  Sim- 
ple crystals  are  usually  tabular  parallel  to  00  P  06  (100),  which  is  striated 
vertically.  Consequently,  in  twins  this  face  exhibits  striatums  arranged 
feather-like.    The  angle  no  A  iTo=50°  21'. 

The  cleavage  of  chrysoberyl  is  distinct  parallel  to  Poo  (on),  and 
indistinct  parallel  to  00  P  06  (010)  and  00  P  65  (100).  Its  fracture  is 
uneven  or  conchoidal.  Its  color  is  some  shade  of  light  green  or  yellow 
by  reflected  light.  It  is  transparent  or  translucent  and  in  some  cases  is 
distinctly  red  by  transmitted  light.     It  is  strongly  pleochroic  in  orange, 


ALUMINATES,   FERRITES,   ETC.  203 

green  and  red  tints.    The  mineral  is  brittle,  has  a  hardness  of  8.5  and  a 
density  of  about  3.6.     Its  refractive  indices  are:  «  =  1.7470,  (3=1.7484, 

T=  I-7565- 

Four  distinct  varieties  are  recognized: 

Ordinary,  pale  green,  translucent. 

Gem,  yellow  and  transparent. 

Alexandrite,  emerald-green  in  color,  but  red  by  transmitted  light, 
transparent,  usually  in  twins.    Used  as  a  gem. 

Cat's-eye,  a  greenish  variety  exhibiting  a  play  of  colors  (chatoyancy). 

Before  the  blowpipe  the  mineral  is  infusible.  It  yields  the  Al  reac- 
tion with  Co(N03)2,  but  otherwise  is  only  slightly  affected  by  the  flame. 
It  is  insoluble  in  acids. 

Chrysoberyl  is  characterized  by  its  crystallization  and  great  hard- 


Fig.  99.  I-'k;.  100.  Fie 


Fig.  oo.—Chrysolicryl  Crystal  with  «  1*«  ,  100  («);   «  I*  w  ,010  (*);    «P!,i2o(s); 

aP»,  121  Mi  P,  in  (•) and  PS, mi  (fl. 

Flu.  100.— Chrysoberyl  Twinned  about  jp  5  (0.11)- 

Fig.  101. — Chrysoberyl  Pscudohexagona]  Sibling  Twinned  about  3P  w  {031). 

ness.  It  most  closely  resembles  the  beryllium  silicate,  beryl,  in  appear- 
ance, but  is  easily  distinguished  from  this  by  its  crystallization. 

Synthesis. — Crystals  have  been  made  by  fusing  BeO  and  AI2O3 
with  boric  acid  and  then  distilling  off  the  boric  acid. 

Occurrence  and  Origin. — Chrysoberyl  is  found  principally  in  granites 
and  crystalline  schists  and  as  grains  in  the  sands  produced  by  the  erosion 
of  these  rocks.  In  its  original  position  the  mineral  is  a  separation 
from  the  magma  that  produced  the  rocks. 

Localities. — Its  best  known  localities  are  in  Minas  Geraes,  Brazil; 
near  Ekaterinburg,  Ural;  in  the  Moume  Mrs.,  Ireland;  at  Haddam, 
Conn.;  at  Greenfield,  N.  Y.;  at  Orange  Summit,  N.  Hamp.;  and  at 
Norway  and  Stoneham,  Me.  The  alexandrite  comes  from  Ceylon,  where 
it  occurs  as  pebbles,  and  from  the  Urals. 


204 


DESCRIPTIVE  MINERALOGY 


Braunite  (MnMnOs)  occurs  massive  and  in  crystals.  The  latter 
are  tetragonal  (ditetragonal  bipyramidal  class),  with  a  :  c=i  :  .9922. 
They  are  usually  simple  bipyramids  P(ni).  Because  of  the  nearly 
equal  value  of  a  and  c  all  crystals  are  isometric  in  habit.  The  angle 
iiiAi^i  •  700  7'.  Twins  are  common,  with  P  00(101)  the  twinning 
plane.      Cleavage  is  perfect  parallel  to  P(ni). 

The  mineral  is  brownish  black  to  steel-gray  in  color  and  in  streak. 
Its  luster  is  submetallic.  Its  hardness  is  6-6.5  and  density  4.7.  It  is 
infusible  before  the  blowpipe.  With  fluxes  it  gives  the  usual  reactions 
for  manganese.    It  is  soluble  in  HC1  yielding  chlorine. 

It  occurs  in  veins  with  manganese  and  other  ores  in  Piedmont,  Italy, 
and  at  Pajsberg  and  various  other  places  in  Sweden,  where  its  origin 
is  secondary. 

Hausmannite  (Mn2Mn04)  crystallizes  like  braunite,  but  a  :  c— 
1  :  1. 1573  and  its  crystals  are,  therefore,  distinctly  tetragonal  in  habit. 
They  are  usually  simply  P(ni)  or  combinations  of  P(in)  and  JP(ii3), 
though  much  more  complicated  crystals  are  known.  The  angle 
in  A  iTi=6o°  1'.     Twins  and  fourlings  (Fig.  102)  are  common,  with 


A  B 

Fig.  102. — Hausmannite.     (A)  Simple  Crystal,  P,  in   (p)  and  oP,  001  (c). 

Fiveling  Twinned  about  P  °o  (101). 


(B) 


P  00(101)  the  twinning  plane.  The  cleavage  is  imperfect  parallel  to 
oP(ooi).     The  mineral  also  occurs  in  granular  masses. 

Hausmannite  is  brownish  black.  Its  streak  is  chestnut  brown. 
Its  hardness  is  5-5.5  and  density  4.8.  Its  reactions  are  the  same  as 
those  of  braunite. 

Hausmannite  occurs  as  crystals  at  Ilmenau,  Thiiringia;  Ilfeld, 
Harz,  and  as  granular  masses  in  dolomite  at  Nordmark  and  several 
other  points  in  Sweden.  Like  braunite  it  is  probably  a  decomposition 
product  of  other  manganese  minerals. 


CHAPTER  X 
THE  NITRATES  AND  BORATES 

THE  NITRATES 

The  nitrates  are  salts  of  nitric  acid.  Only  two  are  of  importance 
to  us,  saltpeter  (KNO3)  and  chile  saltpeter  (NaNCfe).  Both  are  color- 
less, or  white,  crystalline  bodies,  both  are  soluble  in  water  and  both  pro- 
duce a  cooling  taste  when  applied  to  the  tongue.  The  potassium  com- 
pound is  distinguished  from  the  sodium  compound  by  the  flame  test. 
Both  minerals  when  heated  in  the  closed  tube  with  KHSO4  yield  red 
vapors  of  nitrogen  peroxide  (NO2). 

Soda  Niter  (NaN03) 

Soda  niter,  or  chile  saltpeter,  is  usually  in  incrustations  on  mineral 
surfaces  or  in  massive  forms.  It  consists  of  63.5  per  cent  N2O5  and 
36.5  per  cent  Na20. 

Its  crystals  are  in  the  ditrigonal  scalenohedral  class  of  the  hexagonal 
system  with  an  axial  ratio  of  a  :  e=  1  :  .8297.  They  are  usually  rhom- 
bohedrous  R(ioTi)  in  some  cases  modified  by  oR(oooi).  Apparently 
the  mineral  is  completely  isomorphous  with  calcite  (CaCOs). 

Its  cleavage  is  perfect  parallel  to  the  rhombohedron.  Its  hardness 
is  under  2,  its  density  about  2.27  and  its  melting  point  about  31 20. 
Its  luster  is  vitreous;  color  white,  or  brown,  gray  or  yellow.  The  min- 
eral is  transparent.  Its  refractive  indices  for  yellow  light  are:  to  =  1 .5854, 
€=1.3369. 

Soda  niter  deflagrates  when  heated  on  charcoal  and  colors  the  flame 
yellow.  When  exposed  to  the  air  it  attracts  moisture  and  finally  lique- 
fies.   It  is  completely  soluble  in  three  times  its  own  weight  of  water. 

Occurrence  and  Localities. — The  principal  occurrences  of  the  mineral 
are  in  the  district  of  Tarapaca,  northern  Chile,  where,  mixed 
with  the  iodate  and  other  salts  of  sodium  and  potassium,  under  the 
name  calichey  it  comprises  beds  several  feet  thick  on  the  surface  of  rain- 
less pampas,  and  in  Bolivia  at  Arane  under  the  same  conditions.  It  is 
associated  with  gypsum,  salt   and    other  soluble  minerals.    Smaller 

205 


206  DESCRIPTIVE   MINERALOGY 

deposits  are  found  in  Humboldt  Co.,  Nevada,  in  San  Bernardino  Co., 
Cal.,  and  in  southern  New  Mexico. 

The  material  is  thought  to  result  from  the  action  of  microorganisms 
upon  organic  matter  decomposing  in  the  presence  of  abundant  air. 

Uses. — Soda  niter  is  used  in  the  production  of  nitric  acid,  and  in  the 
manufacture  of  fertilizers  and  gunpowder.  About  480,000  tons  are 
imported  into  the  United  States  annually  at  a  cost  of  $15,430,000. 
Most  of  it  comes  from  Chile. 

Since  soda  niter  usually  contains  sodium  iodate  as  an  impurity,  the 
mineral  is  an  important  source  of  iodine. 

Niter  (KNO3) 

Niter,  or  saltpeter,  resembles  soda  niter  in  appearance.  It  gener- 
ally occurs  in  crusts,  in  silky  tufts  and  in  groups  of  acicular  crystals. 
Its  crystals  are  orthorhombic  with  a  :  b  :  £=.5910  :  1  :  .7011.  Their 
habit  is  hexagonal.  The  principal  forms  observed  on  them  are  00  P(i  10), 
00  Poo  (100),  00  P 66  (010),  oP(ooi),  P(in),  and  a  series  of  brachy- 
domes.  In  many  respects  the  mineral  is  apparently  isomorphous  with 
aragonite  which  is  the  orthorhombic  dimorph  of  calcite.  At  1260  it 
passes  over  into  an  hexagonal  (trigonal)  form.  Its  cleavage  is  perfect 
parallel  to  P  66  (on).  Its  fracture  is  uneven;  its  hardness  2  and  den- 
sity 2.1.     Its  medium  refractive  index  for  yellow  light,  /3=  1.5056. 

Niter  deflagrates  more  violently  than  soda  niter  and  detonates  with 
combustible  substances.  It  fuses  at  about  335 °.  It  colors  the  blowpipe 
flame  violet.     It  is  soluble  in  water. 

Occurrence  and  Localities. — The  mineral  forms  abundantly  in  dry 
soils  in  Spain,  Egypt,  Persia,  Ceylon  and  India,  where  it  is  produced 
by  a  ferment,  and  on  the  bottoms  of  caves  in  the  limestones  of  Madison 
Co.,  Ky.,  of  Tennessee,  of  the  valley  of  Virginia  and  of  the  Mississippi 
Valley. 

Production. — Most  of  the  niter  used  in  the  arts  is  manufactured,  but 
some  is  obtained  from  the  deposits  in  Ceylon  and  in  India.  The 
amount  imported  in  191 2  aggregated  6,976,000  lb.,  valued  at  $226,851. 

THE  BORATES 

The  borates  are  salts  of  boric  acid,  H3BO3,  metaboric  acid,  HBO2, 
tetraboric  acid,  H2B4O7,  hexaboric  acid,  H4B6O11,  and  various  poly- 
boric  acids  in  which  boron  is  present  in  still  larger  proportion.  The 
metaacid  is  obtained  from  the  orthoacid  by  heating  at  ioo°,  at  which 


NITRATES  AND  BORATES 


207 


temperature  the  former  loses  one  molecule  of  water,  thus:  H3BO3  — 
H20=HB02,  and  the  tetraacid  by  heating  the  same  compound  to  1600 
at  which  temperature  5  molecules  of  water  are  lost  from  4  molecules  of 
the  acid,  thus:  4H3BO3— 5H2O =1128407.  Hexaboric  acid  may  be 
regarded  as  the  orthoacid  less  1$  molecules  of  water,  thus:  6H3BO3 
-7H2O=H4B0Oii. 

Only  three  of  the  borates  are  important  enough  to  be  discussed 
here.  These  are  borax,  a  sodium  tetraborate  (Na2B407*ioH20),  cole- 
manite,  a  hexaborate  (Ca2BeOn-5H20)  and  boracite,  a  magnesium 
chloro-polyborate  (Mg5(MgCl)2Bi603o).  Borax  and  colemanite  are 
commercial  substances  that  are  produced  in  large  quantities. 

All  borates  and  many  other  compounds  containing  boron  when 
pulverized  and  moistened  with  H2SO4  impart  an  intense  yellow-green 
color  to  the  flame.  If  boron  compounds  are  dissolved  in  hydrochloric 
acid,  the  solution  will  turn  turmeric  paper  reddish  brown  after  drying 
at  ioo°.  The  color  changes  to  black  when  the  stain  is  treated  with 
ammonia. 


% 


m 


Borax  (Na2B407  10H2O) 

Borax  occurs  as  crystals  and  as  a  crystalline  cement  between  sand 
grains  around  salt  lakes,  as  an  incrustation  on  the  surfaces  of  marshes 
and  on  the  sands  in  desert  regions,  and  dissolved 
in  the  water  of  certain  lakes  in  deserts.  It 
occurs  also  as  bedded  deposits  interlayered  with 
sedimentary  rocks. 

The  composition  of  borax  is  16.2  per  cent 
Na20, 36.6  per  cent  B2O3  and  47.2  per  cent  H2O. 

Crystals  are  monoclinic  (prismatic  class),  with 
alb:  c=  1.0995  :  x  :  5629,  and  £=73°  25'. 
They  are  prismatic  in  habit  and  in  general  form 
resemble  very  closely  crystals  of  pyroxene. 
The  principal  planes  occurring  on  them  are 
ooPoo(ioo),  ooP(no),  oP(ooi),  — P(m)  and 
—  2P(22i)  (Fig.  103).  Their  cleavage  is  perfect 
parallel   to  00  P  00  (100),  and  their  fracture  conchoidal.     The  angle 

iioaiTo=93°- 

The  mineral  has  a  white,  grayish  or  bluish  color  and  a  white  streak. 

It  is  brittle,  vitreous,  resinous  or  earthy;  is  translucent  or  opaque;  has 

a  hardness  of  2-2.5, a  density  of  1. 69-1. 72,  and  a  sweetish  alkaline  taste. 

On  exposure  to  the  air  the  mineral  loses  water  and  tends  to  become  white 


Fig.  103. — Borax  Crystal 
with  00  P,  no  (m); 
00  P  00  ,  100  (a);  00  P  5b  , 
010  (b);  oP,  001  (c); 
P,  In  (0)  and  2P,  221 

W- 


208  DESCRIPTIVE  MINERALOGY 

and  opaque,  whatever  its  color  in  the  fresh  condition.  Its  medium 
refractive  index  for  yellow  light,  0=  1.4686. 

Before  the  blowpipe  borax  puffs  up  and  fuses  to  a  transparent 
globule.  Fused  with  fluorite  and  potassium  bisulphate  it  colors 
the  flame  green.  It  is  soluble  in  water,  yielding  a  weakly  alkaline 
solution. 

Occurrence. — The  principal  method  of  occurrence  of  the  mineral  is 
as  a  deposit  from  salt  lakes  in  arid  regions,  and  as  incrustations  on  the 
surfaces  of  alkaline  marshes  overlying  buried  borax  deposits.  The 
original  beds  were  deposited  by  the  evaporation  to  dryness  of  ancient 
salt  lakes,  and  the  incrustations  were  produced  by  the  solution  of  these 
deposits  by  ground  water,  and  the  rise  of  the  solutions  to  the  surface  by 
capillarity. 

Localities. — Borax  occurs  in  the  water  of  salt  lakes  in  Tibet;  of 
several  small  lakes  in  Lake  County,  and  of  Borax  Lake  in  San  Bernardino 
County  in  California,  and  in  the  mud  and  marshes  around  their  borders. 
It  occurs  also  in  the  sands  of  Death  Valley  in  the  same  State,  and  in 
various  marshes  in  Esmeralda  County,  Nevada.  Other  large  deposits 
are  found  in  Chile  and  Peru. 

Uses. — Borax  is  used  as  an  antiseptic,  in  medicine,  in  the  arts  for 
soldering  brass  and  welding  mqf  als,  and  in  the  manufacture  of  cosmetics. 

« 

Boric  acid  obtained  from  borax  and  colemanite  is  employed  in  the 
manufacture  of  colored  glazes,  in  making  enamels  and  glass,  as  an 
antiseptic  and  a  preservative.  Some  of  the  borates  are  used  as  pig- 
ments. 

Production. — Borax  was  formerly  obtained  in  the  United  States, 
especially  in  California,  Oregon  and  Nevada,  by  the  evaporation  of 
the  water  of  borax  lakes,  by  washing  the  crystals  from  the  mud  on  their 
bottoms  and  by  the  leaching  of  the  mineral  from  marsh  soil.  At  pres- 
ent, however,  nearly  all  the  borax  of  commerce  is  manufactured  from 
colemanite. 

Colemanite  (Ca2B60n  5H2O) 

Colemanite  occurs  in  crystals  and  in  granular  and  compact  masses. 
It  is  the  source  of  all  the  borax  now  manufactured  in  the  United  States. 

The  formula  ascribed  to  the  mineral  corresponds  to  27.2  per  cent 
CaO,  50.9  per  cent  B2O3  and  21.9  per  cent  H2O.  As  usually  found, 
however,  it  contains  a  little  MgO  and  SK>2.  A  crystal  from  Death 
Valley,  California,  yielded: 

8203  =  50.70;  CaO=27.3i;  MgO=.io;  H20=2i.87.    Total  =  99-98. 


NITRATES  AND  BORATES 


209 


The  mineral  crystallizes  in  the  monoclinic  system  (prismatic  class), 
in  short,  prismatic  crystals  (Fig.  104),  with  the  axial  constants  a  :  b  :  c 
=  .7769  :  1  :  .5416  and  £=69°  43'.  The  crystals  are  usually  rich  in 
forms.  Their  cleavage  is  perfect  parallel  to  00  O  00  (010),  and  less 
perfect  parallel  to  oP(ooi).  Their  fracture  is  uneven.  The  angle 
110A  110=72°  4'. 

Colemanite  is  colorless,  milky  white,  yellowish  white  or  gray.  It 
is  transparent  or  translucent,  has  a  vitreous  or  adamantine  luster,  a 
hardness  of  4  to  4.5  and  a  specific 
gravity  of  2.4.  Its  index  of  refrac- 
tion for  yellow  light,  0=  1.5920. 

Before  the  blowpipe  it  decrep- 
itates, exfoliates,  and  partially 
fuses,  at  the  same  time  coloring 
the  flame  yellowish  green.  It  is 
soluble  in  hot  HC1,  but  from  the 
solution  upon  cooling  a  volumi- 
nous mass  of  boric  acid  separates 
as  a  white  gelatinous  precipitate.  Fig.  104. — Colemanite  Crystals  with  «  p, 

It  is  easily  distinguished  from      IIO(m>;  3P  * ,  301  (w);  «  P « ,  100  (a); 


m 


m 


ooPjp,,oio  (6);  oP,  001  (c)\  — P,  in 
(0);  2P«,o2i(a);  Pob,oii(*c);  co  P2, 
210  (/);  2P06,  201  (A);  2P,  221  (*)  and 
P,  I"  (y). 


other  white  translucent  minerals, 
except  those  containing  boron, 
by  the  flame  test.  It  is  distin- 
guished from  borax  by  its  insolu- 
bility in  water  and  from  boracite  by  its  inferior  hardness  and  crystal- 
lization. 

Synthesis. — Colemanite  has  been  prepared  by  treating  ulexite 
(NaCaB509  •  8H2O)  with  a  saturated  solution  of  NaCl  at  700. 

Occurrence  and  Origin. — The  mineral  occurs  as  indefinite  layers 
interstratified  with  shale  and  limestones  that  are  associated  with  basalt. 
The  rocks  contain  layers  and  nodules  of  colemanite.  Gypsum  is  often 
associated  with  the  borate  and  in  some  places  is  in  excess.  The  cole- 
manite is  believed  to  be  the  result  of  the  action  of  emanations  from 
the  basalt  upon  the  limestone. 

Localities. — Colemanite  occurs  in  Death  Valley,  California,  near 
Daggett,  San  Bernardino  County,  and  near  Lang  Station,  Los  Angeles 
County,  and  at  other  points  in  the  same  State,  and  in  western  Nevada, 
near  Death  Valley.  A  snow-white,  chalky  variety  (priceite)  has  been 
found  in  Curry  County,  Oregon,  and  a  compact  nodular  variety  (pander- 
mite)  at  the  Sea  of  Marmora,  and  at  various  points  in  Asia  Minor. 

Preparation. — Colemanite  is  at  the  present  time  the  principal  source 


210  DESCRIPTIVE  MINERALOGY 

of  borax.  The  crude  material  as  mined  contains  from  5  per  cent  to  35 
per  cent  of  anhydrous  boric  acid  (B2O3).  This  is  crushed  and  roasted. 
The  colemanite  breaks  into  a  white  powder  which  is  separated  from 
pieces  of  rock  and  other  impurities  by  screening,  and  then  is  bagged  and 
shipped  to  the  refineries  where  it  is  manufactured  into  borax  and  boracic 
acid. 

Production. — The  principal  mines  producing  the  mineral  in  191 2 
were  situated  in  the  Death  Valley  section  of  Inyo  County,  near  Lang 
Station  in  Los  Angeles  County,  California,  and  in  Ventura  County  in 
the  same  State.  The  total  production  during  the  year  was  42,315 
tons  of  crude  ore,  valued  at  $1,127,813.  The  imports  of  crude  ore, 
refined  borax  and  boric  acid  during  the  same  year  were  valued  at  $11,200. 
The  production  of  the  United  States  in  boron  acid  compounds  is 
about  half  that  of  the  entire  world,  with  Chile  producing  nearly  all 
the  rest. 

Boracite  (Mg5(MgCl)2Bi603o) 

Boracite  is  interesting  as  a  mineral,  the  form  and  internal  structure 
of  which  do  not  correspond,  that  is,  do  not  possess  the  same  symmetry. 
Its  crystals  have  the  well  marked  hextetrahedral  symmetry  of  the  iso- 
metric system,  but  their  internal  structure,  as  revealed  by  their  optical 
properties  is  orthorhombic.  This  is  due  to  the  fact  that  the  substance 
is  dimorphous.  Above  265  °  it  is  isometric  and  below  that  temperature 
orthorhombic.  Crystals  formed  at  temperatures  above  265°  assume 
the  isometric  shapes.  As  the  temperature  falls  the  substance  changes 
to  its  orthorhombic  form,  and  there  results  a  pseudomorph  of  ortho- 
rhombic boracite  after  its  isometric  dimorph. 

It  is  a  salt  of  the  acid  which  may  be  regarded  as  related  to  boric 
acid  as  follows:  8H3BO3— gl^C^HeBgOis.  Ten  atoms  of  hydrogen 
in  two  molecules  of  the  acid  are  replaced  by  Mgs  and  the  other  two  by 
2(MgCl).  The  resulting  combination  is:  31.4  per  cent  MgO;  7.9  per 
cent  CI  and  62.5  per  cent  B203=ioi.8(0=C1=i.9).  The  mineral 
alters  slowly,  taking  up  water,  so  that  some  specimens  yield  water  on 
analysis  and  in  the  closed  tube  (stassfurti'.e  and  parasite). 

The  forms  usually  found  on  the  crystals  are  -(in),    ooO(no), 

•  2 

00  O  00  (100), (1T1)  (Fig.  105).     Usually  the  positive  and  negative 

2 

tetrahedrons  may  be  distinguished  by  their  luster,  the  faces  of  the  posi- 
tive form  being  brilliant  and  those  of  the  negative  form  dull.  The 
crystals  are  isolated,  or  embedded,  and  rarely  in  groups.    They  are 


NITRATES  AND  BORATES 


211 


Fig.  105.  —  Boracite 
Crystal  with  00  O  00  , 
100  (a);    00  O,  no  (rf); 

O  O 

+-,  in   (0)  and  --, 

ill  (Oi). 


strongly  pyroelectric  with  the  analogue  pole  in  the  negative  tetrahedrons. 
The  mineral  is  also  found  massive. 

Boracite  is  transparent  or  translucent  and  is  gray,  yellow,  or  green. 
Its  streak  is  white.  Its  luster  is  vitreous.  Its  cleavage  is  indistinct 
parallel  to  O(ni)  and  its  fracture  is  conchoidal. 
The  mineral  is  brittle.  Its  hardness  is  7  and 
its  density  3.  Its  refractive  index  0,  for  yellow 
light,  =1.667. 

Boracite  fuses  easily  before  the  blowpipe 
with  intumescence  to  a  white  pearly  mass,  at 
the  same  time  coloring  the  flame  green.  With 
copper  oxides  it  colors  the  flame  azure-blue. 
When  moistened  with  Co(NC>3)2  it  gives  the 
pink  reaction  for  magnesium.  Some  massive 
forms  yield  water  in  the  closed  tube,  in  conse- 
quence of  weathering.  The  mineral  is  soluble 
in  HC1. 

Boracite  is  distinguished  from  other  boron  salts  by  its  crystallization, 
its  lack  of  cleavage  and  its  much  greater  hardness.  The  massive  vari- 
eties which  resemble  fine-grained  white  marble  can  be  distinguished 
from  this  by  the  flame  coloration,  hardness  and  reaction  with  HC1. 

Syntheses. — Crystals  have  been  formed  by  heating  borax,  MgCb 
and  a  little  water  at  2750,  and  by  fusing  borax  with  a  mixture  of  NaCl 
and  MgCb- 

Occurrence. — Boracite  occurs  in  beds  with  anhydrite,  gypsum  and 
salt,  and  as  crystals  in  metamorphosed  limestones. 

Localities. — It  is  found  as  crystals  in  gypsum  and  anhydrite  at 
Ltineburg,  Hanover,  and  Segeberg,  Holstein;  in  carnallite  at  Stassfurt, 
Prussia;  and  in  radiating  nodules  (stassfurtite)  and  in  massive  layers 
associated  with  salt  beds  at  the  last-named  locality.  It  is  rare  in  the 
United  States. 

Uses  and  Production. — Boracite  is  utilized  in  Europe  as  a  source  of 
boron  compounds.    Turkey  produces  annually  about  12,000  tons. 


CHAPTER  XI 
THE   CARBONATES 

The  carbonates  constitute  an  important,  though  not  a  very  large, 
group  of  minerals,  though  one  of  them,  calcite,  is  among  the  most  com- 
mon of  all  minerals.  They  are  all  salts  of  carbonic  acid  (H2CO3).  Those 
in  which  all  the  hydrogen  has  been  replaced  by  metal  are  normal  salts, 
those  in  which  the  replacement  has  been  by  a  metal  and  a  hydroxyl 
group  are  basic  salts.    Both  groups  are  represented  by  common  minerals. 

The  normal  salts  include  both  anhydrous  salts  and  salts  combined 
with  water  of  crystallization.  Illustrations  of  the  three  classes  of  car- 
bonates are:  CaCC>3,  calcite,  normal  salt;  Na2C03 •  10H2O,  soda, 
hydrous  salt  and  (Cu '011)2003,  malachite,  basic  salt.  All  carbonates 
effervesce  in  hot  acids.  The  basic  salts  yield  water  at  a  high  tempera- 
ture only;  the  hydrous  ones  at  a  low  temperature. 

The  carbonates  are  all  transparent  or  translucent,  and  all  are  poor 
conductors  of  electricity.    Most  of  them  are  practically  nonconductors. 

ANHYDROUS  CARBONATES 

NORMAL  CARBONATES 

The  anhydrous  normal  carbonates  comprise  the  most  important 
carbonates  that  occur  as  minerals.  Most  of  them  are  included  in  a 
single  large  group  whose  members  are  dimorphous,  crystallizing  in  the 
ditrigonal  scalenohedral  class  of  the  hexagonal  system  and  in  the  holo- 
hedral  division  (rhombic  bipyramidal  class)  of  the  orthorhombic  sys- 
tem. The  calcium  carbonate  exists  in  three  forms  but  only  two  are 
known  to  occur  as  minerals. 

CALCITE-ARAGONITE   GROUP 

The  relation  of  the  dimorphs  of  this  group  to  one  another  has  been 
subjected  to  much  study,  especially  with  reference  to  the  two  forms  of 
CaC03.  The  orthorhombic  form,  aragonite,  passes  into  the  hexagonal 
form,  calcite,  upon  heating  to  about  4000.  At  all  temperatures  below 
9700,  calcite  is  the  stable  form.  Moreover,  while  calcite  crystallizes 
from  a  dilute  solution  of  CaC03  in  water  containing  CP2  at  a  low  tem- 

212 


CARBONATES  213 

perature,  aragonite  separates  at  a  temperature  approaching  that  of 
boiling  water — the  more  freely,  the  less  CO2  in  the  solution.  Arag- 
onite crystals  will  also  separate  from  a  solution  of  calcium  carbonate, 
if,  at  the  same  time,  it  contains  a  grain  of  an  orthorhombic  carbonate, 
or  a  small  quantity  of  a  soluble  sulphate.  Some  of  the  other  carbon- 
ates, for  instance,  strontianite  (the  orthorhombic  SrCOs),  pass  over 
into  an  hexagonal  form  like  that  of  calcite  at  700°,  but  again  change 
to  the  orthorhombic  form  upon  cooling.  For  convenience  the  group 
is  divided  for  discussion  into  the  calcite  division  and  the  aragonite 
division. 

CALCITE  DIVISION 

The  calcite  division  of  carbonates  includes  nine  or  more  distinct 
compounds  and  a  number  of  well  defined  varieties  of  these.  Six  of  the 
compounds  are  common  minerals.  All  crystallize  in  the  ditrigonal 
scalenohedral  class  of  the  hexagonal  system  and  are  thus  isomorphous. 
Their  most  common  crystals  have  a  rhombohedral  habit.  The  names  of 
the  six  common  members  with  their  axial  ratios  are: 

Calcite  CaCCto  0 : c=  1  :  .8543 

Magnesite  MgCC>3  =1  :  .8095 

Siderite  FeCC>3  =1  :  .§191 

Rhodochrosite  MnCC>3  =1  •   8259 

Smit'.iSonitc  ZnCCfe  =1  :  .8062 

There  is  usually  also  included  in  the  group  the  mineral  dolomite,  which 
is  a  calcium  magnesium  carbonate  in  which  CaCCfe  and  MgCC>3  are 
present  in  the  molecular  proportions,  thus:  MgCCb-CaCOs,  or 
MgCa(C03)2.  Its  crystals  are  similar  to  those  of  calcite  and  its  physical 
properties  are  intermediate  between  those  of  calcite  and  magnesite. 
Its  symmetry,  however,  as  revealed  by  etching  is  tetartohedral  (rhom- 
bohedral class). 

The  close  relationship  existing  between  the  members  of  the  group 
(including  dolomite)  will  be  appreciated  upon  comparing  the  data  in 
the  following  table: 

H 

Calcite 3. 

Dolomite 3-5~4 

Magnesite 3-5~4-5 

Siderite 3-5"4 

Rhodochrosite..  3.5-4.5 

Smithsonite. ...  5. 


Ref.  Indices 

Sp.  Gr. 

a :  c 

ion  Aon 

I     (1) 

c 

2-73 

■8543 

74°  55' 

1-6585 

1.4863 

2.85 

.8322 

73°  45' 

I. 6817 

1 . 5026 

3-°4 

.8095 

72°  36' 

1. 717 

I-5I5 

3-88 

.8191 

73°  0' 

1.8724 

16338 

3  55 

•8259 

73   0 

1.820 

1  5973 

4-45 

.8062 

720  20' 

i.8i8± 

1. 6177 

214 


DESCRIPTIVE  MINERALUGY 


Calcite  (CaC03) 

Calcite  is  one  of  the  most  beautifully  jcrystallized  minerals  known. 
Its  crystals  are  very  common,  and  sometimes  very  large.  They  are 
usually  colorless,  though  sometimes  colored,  and  are  nearly  always 
transparent.  Besides  occurring  in  crystals  the  mineral  is  often  found 
massive,  in  granular  aggregates,  in  stalactites,  in  pulverulent  masses, 


Fig.  106. 


e 


m 


Fig.  108. 


m 


Fig.  107.  Fig.  109. 

Fig.  106.— Calcite  Crystal  with  — JR,  01T2  (<?)  and  00  R;  10T0  (m).    Nail-head  Spar. 

Fig.  107. — Calcite  Crystal  with  m  and  e.    Prismatic  Type. 

Fig.  108.— Calcite  Crystals  with  m;  R3,  2 131  (v)  and  R,  10T1  (r).    Dog-tooth  Spar. 

Fig.  109. — Calcite  with  r,  v;  4R,  4041  (M)  and  R*,  3251  (y). 

in  radial  groupings,  in  fibrous  masses  and  in  a  variety  of  other  forms.  As 
calcite  is  soluble  in  water  containing  CO2,  it  has  often  been  found  pseu- 
domorphing  other  minerals. 

Theoretically,  calcite  contains  56  per  cent  CaO  and  44  per  cent  CO2, 
but  practically  the  mineral  contains  also  small  quantities  of  Mg,  Fe, 
Mn,  Zn  and  Pb,  metals  whose  carbonates  are  isomorphous  with 
CaC03. 

The  forms  that  have  been  observed  on  calcite  crystals  are  arranged 


CARBONATES 


215 


in  such  a  manner  as  to  produce  three  distinct  types  of  habit,  as  fol- 
lows: (i)  the  rhombohedral  type,  bounded  by  the  flat  rhombohedrons, 
R(ioTi),  — JR(oiTa)  and  often  blunt  scale- 
nohedrons,  like  R3(2ili)  and  ^(3145) 
in  which  the  rhombohedrons  predominate 
(Fig.  106);  (2)  the  prismatic  type,  with 
the  prism  00  P(ioTo)  predominating,  and 
—  jR(oiia)  as  the  principal  termination 
(Fig.  107),  and  (3)  dog-tooth  spar,  contain- 
ing the  same  scalenohedrons  as  on  the  first 
type  mentioned  above  with  other  steeper 
ones  and  small  steep  rhombohedral  planes 
(Fig.  108,  109,  no).  Nail-head  spar  con- 
tains the  flat  rhombohedron  —  JR(oiTa) 
with  the  prism  00  P(ioTo)  (Fig.  106). 

Some  of  the  crystals  are  very  compli- 
cated, belonging  to  no  one  of  the  distinct 

types  described  above,  but    forming  barrel-shaped  or  almost   round 
bodies.    Over  300  well  established  forms  have  been  identified  on  them. 

Twins  are  common.  The  principal  laws  are:  (1)  twinning  plane 
oP(oooi),  with  the  vertical  axis  common  to  the  twinned  parts  (Fig. 
in),  (2)  twinning  plane  —  §R(otl2),  with  the  two  vertical  axes  inclined 


Fie.  1 10.— Prismatic  Crystals 
of  Calrite  Terminated  by 
Scalenohedrons  and  Rhom  - 
bohedrons,  from  Cumber- 
laud,  England. 


Fic.  in. — Calcite,  R*  (.3131)  Twinned  about  oP  (0001). 
— Calcite:    Twin  and  Per/synthetic  Trilling  of  R  (10T1)  about  —  )R  (01 


at  an  angle  of  about  525°  (Fig.  112)  and  (3)  twinning  plane  R(ioTi), 
with  the  vertical  axes  inclined  8o°  14'  (Fig.  113). 

Twins  of  the  second  class  can  easily  be  produced  artificially  on  cleav- 
age rhombs  by  pressing  a  dull  knife  edge  on  the  obtuse  rhombohedral 
edge  with  sufficient  force  to  move  a  portion  of  the  mass  (Fig.  114). 
The  change  of  position  of  a  portion  of  the  calcite  does  not  destroy  its 


DESCRIPTIVE   MINERALOGY 


transparency  in  the  least.     Repeated  twinning  of  this  kind  is  frequently 
seen  in  marble  (Fig.  115),  where  it  gives  rise  to  parallel  lamellae. 

The  cleavage  of  calcite  is  so  perfect  parallel  to  R  that  crystals  when 


Fig. 


Fig.  113.  F10.  114. 

Fig.  113— Calcite  with  m,  c and  e,  Twinned  about  R  (ioii). 
14.— Artificial  Twin  of  Calcite,  with  —  JR  (011a)  the  Twinning  Plane. 


shattered  by  a  hammer  blow  usually  break  into  perfect  little  rhombo- 
hedrons.  Its  hardness  is  about  3  and  its  density  2.713.  Pure  calcite 
is  colorless  and  transparent,  but  most  specimens  are  white  or  some  pale 
shade  of  red,  green,  gray, 
blue,  yellow,  or  even  brown 
or  black  when  very  impure, 
and  are  translucent  or  opaque. 
The  mineral  is  very  strongly 
doubly  refracting,  (see  p.  213). 
It  is  a  very  poor  conductor  of 
electricity. 

The  principal  varieties  of 
the  mineral  to  which  distinct 
names  have  been  given  are: 

Iceland  spar,  the  trans- 
parent variety  used  in  the 
manufacture  of  optical  instru- 
ments. 

Salin  spar,  a  fine,  fibrous 
variety  with  a  satiny  luster. 

Limestone,     granular    ag- 
gregates   occurring    as    rock 
masses. 
Marble,  a  crystalline  limestone,  showing  when  broken  the  cleavage 
faces  of  the  individual  crystals. 

Lithographic  stone.  1  very  fine  and  even-grained  limestone. 


Fig.  115. — Thin  Section  of  Marble  Viewed  by 
Polarized  Light.  The  dark  bars  are  poly- 
synthetic  twinning  lamellae.  Magnified  5 
diameters. 


CARBONATES  217 

Stalactites,  cylinders  or  cones  of  calcite  that  hang  from  the  roofs  of 
caves.    They  are  formed  by  the  evaporation  of  dripping  water. 

Stalagmites,  corresponding  cones  on  the  floors  of  caves  beneath  the 
stalactites. 

Mexican  onyx,  banded  crystalline  calcite,  often  transparent. 
Usually  portions  of  stalactites. 

Travertine,  a  deposit  of  white  or  yellow  porous  calcite  produced 
in  springs  or  rivers,  often  around  organic  material  like  the  blades 
or  roots  of  grass. 

Chalk,  a  fine-grained,  pulverulent  mass  of  calcite,  occurring  in 
large  beds. 

In  the  closed  tube  calcite  often  decrepitates.  Before  the  blowpipe  it 
is  infusible.  It  colors  the  flame  reddish  yellow  and  after  heating  reacts 
alkaline  toward  moistened  litmus  paper.  The  mineral  dissolves  with 
evolution  of  CO2  in  cold  hydrochloric  acid.  Its  dissociation  tempera- 
ture l  is  8980,  though  it  begins  to  lose  CO2  at  a  much  lower  temperature. 

The  reaction  with  HC1,  together  with  the  alkalinity  of  the  mineral 
after  heating,  its  softness  and  its  easy  cleavage,  distinguish  calcite  from 
all  other  minerals.  In  massive  forms  it  has  been  thought  that  it  could 
be  distinguished  from  aragonite  by  heating  its  powder  with  a  little 
Co(NOa)2  solution.  Aragonite  was  thought  to  become  violet-colored 
in  a  few  minutes  while  calcite  remained  unchanged,  but  recent  work 
proves  that  this  test  cannot  be  relied  upon. 

Syntheses. — Calcite  crystals  are  obtained  by  allowing  a  solution 
of  CaCOa  in  dilute  carbonic  acid  to  evaporate  slowly  in  contact  with  the 
air  at  ordinary  temperatures.  If  evaporated  at  from  8o°  to  ioo° 
ordinary  temperatures,  or  in  the  presence  of  a  little  sulphate,  the  ortho- 
rhombic  aragonite  will  form.  Calcite  is  also  formed  by  heating  arag- 
onite to  400-4700. 

Occurrence  and  Origin. — The  mineral  is  widely  distributed  in  beds, 
in  veins  and  as  loose  deposits  on  the  bottoms  of  springs,  lakes  and  rivers. 
Its  principal  methods  of  origin  are  precipitation  from  solutions,  the 
weathering  of  calcareous  minerals,  and  secretion  by  organisms. 

Calcite  is  the  most  important  of  all  pseudomorphing  agencies.  It 
forms  pseudomorphs  after  many  different  minerals  and  the  hard  parts 
of  animals. 

Localities. — The  most  noted  localities  of  crystallized  calcite  are: 
Andreasberg  in  the  Harz;  Freiberg,  Schneeberg  and  other  places  in 
Saxony;  Kapnik,  in  Hungary;  Traverselia,  in  Piedmont;  Alston  Moor 

1  The  dissociation  temperature  of  a  carbonate  is  that  temperature  at  which  the 
pressure  of  the  released  CO?  equals  one  atmosphere. 


218  DESCRIPTIVE  MINERALOGY 

and  Egremont,  in  Cumberland,  Matlock,  in  Derbyshire,  and  the  mines 
of  Cornwall,  England;  Guanajuato,  Mexico;  Lockport,  N.  Y.;  Ke- 
weenaw Point,  Mich.;  the  zinc  regions  of  Illinois,  Wisconsin  and 
Missouri;  Nova  Scotia,  etc. 

Iceland  spar  is  obtained  in  the  Eskefjord  and  the  Breitifjord  in 
Iceland.  Travertine  is  deposited  from  the  waters  of  the  Mammoth 
Hot  Springs,  Yellowstone  National  Park.  It  occurs  also  along  the 
River  Arno,  near  Tivoli,  Rome. 

Uses. — Calcite  has  many  important  uses.  In  the  form  of  Iceland 
spar,  on  account  of  its  strong  double  refraction,  it  is  employed  in  optical 
instruments  for  the  production  of  polarized  light.  Calcite  rocks  are 
used  as  building  and  ornamental  stones.  They  are  employed  also  as 
fluxes  in  smelting  operations,  as  one  of  the  ingredients  in  glass-making 
and  in  the  manufacture  of  lime,  cement,  whiting,  and  in  certain  printing 
operations.    Limestone  is  also  used  as  a  fertilizer. 

Production. — The  calcite  rock  marketed  in  the  United  States  during 
191 2  was  valued  at  about  $44,500,000.  It  was  used  as  follows:  In 
concrete,  $5,634,000;  in  road  and  railroad  making,  $12,000,000;  as  a 
flux,  $10,000,000;  as  building  and  monumental  stone,  $12,800,000; 
in  sugar  factories,  $335,000;  as  riprap,  $1,183,000,  for  paving,  $279,000, 
and  for  other  uses,  $2,400,000.  Moreover,  the  value  of  the  Portland 
cement  manufactured  during  the  year  amounted  to  $67,017,000,  the 
quantity  of  lime  made  to  $13,970,000,  the  value  of  the  hydrated 
lime  to  $1,830,000,  and  of  sand-lime  brick  to  $1,170,884.  The  quantity 
of  limestone  required  for  these  manufactures  is  not  known,  but  it  was 
very  great. 

Magnesite  (MgC03) 

Magnesite  usually  occurs  in  fine-grained  white  masses.  Crystals 
are  rare.  Pure  magnesite  consists  of  52.4  per  cent  CO2  and  47.6  per 
cent  MgO.     It  usually,  however,  contains  some  iron  carbonate. 

Magnesite  is  completely  isomorphous  with  calcite.    Its  cleavage  is 

perfect  parallel  to  R(ioTi).     Its  hardness  is  about  4  and  the  density  3.1. 

The  mineral  is  transparent  or  opaque.     It  varies  in  color  from  white 

to  brown,  but  always  has  a  white  streak.    Its  dissociation  temperature 

is  4450. 

Magnesite  behaves  like  calcite  before  the  blowpipe.  It  effervesces 
in  hot  hydrochloric  acid  and  readily  yields  the  reaction  for  magnesia 
with  Co(N03)2.  It  is  most  easily  distinguished  from  the  latter  mineral 
by  its  density,  by  the  fact  that  it  does  not  color  the  blowpipe  flame  with 
the  yellowish  red  tint  of  calcium  and  does  not  effervesce  in  cold  HC1. 


CARBONATES  219 

Synthesis. — Magnesite  crystals  may  be  obtained  by  heating  MgSOi 
in  a  solution  of  Na2CC>3  at  1600  in  a  closed  tube. 

Occurrence  and  Origin. — Magnesite  usually  occurs  in  veins  and  masses 
associated  with  serpentine  and  other  magnesium  rocks  from  which  it 
has  been  formed  by  decomposition.  It  is  often  accompanied  by  brucite, 
talc,  dolomite  and  other  magnesium  compounds.  It  has  recently  been 
described  as  occurring  also  in  a  distinct  bed  near  Mohave,  Cal.,  inter- 
stratified  with  clays  and  shales.  It  is  thought  that  in  this  case  it  may 
have  been  precipitated  from  solutions  of  magnesium  salts  by  Na2C03. 

Localities. — The  mineral  is  found  abundantly  in  many  foreign  local- 
ities and  at  Bolton,  Mass.;  Bare  Hills,  near  Baltimore,  Md.,  and  in 
Tulare  Co.,  Cal.,  and  near  Texas,  Penn.  The  largest  deposits  are  in 
Greece  and  Hungary. 

Uses. — Magnesite  is  employed  very  largely  in  the  manufacture  of 
magnesite  bricks  used  for  lining  converters  in  steel  works,  in  the  lining 
of  kilns,  etc.,  in  the  manufacture  of  paper  from  wood  pulp,  and  in  mak- 
ing artificial  marble,  tile,  etc.  From  it  are  also  manufactured  epsom 
salts,  magnesia  (the  medicinal  preparation),  and  other  magnesium  com- 
pounds, and  the  carbon  dioxide  used  in  making  soda  water. 

Production. — All  of  the  magnesite  mined  in  the  United  States  comes 
from  California,  where  the  yield  was  10,512  tons  in  191 2,  valued  at 
$105,120.  Most  of  the  magnesite  used  in  the  United  States  is  imported 
from  Hungary  and  Greece.  In  191 2,  14,707  tons  of  crude  material 
entered  the  country  and  125,000  tons  of  the  calcined  product,  the  total 
value  of  which  was  $1,370,000. 

Siderite  (FeC03) 

Siderite  is  an  important  iron  ore,  though  not  as  much  used  as  formerly. 
It  is  found  crystallized  and  massive,  in  botryoidal  and  globular  forms 
and  in  earthy  masses. 

In  composition  the  mineral  is  FeCC>3,  which  is  equivalent  to  62.1 
per  cent  FeO  (48.2  per  cent  Fe)  and  37.9  per  cent  CO2.  Manganese, 
calcite  and  magnesium  are  also  often  present  in  it. 

Crystals  are  more  common  than  those  of  magnesite.  They  fre- 
quently contain  the  basal  plane  and  the  steep  rhombohedrons— 8R(o85i) 
and  —  58.(0551).  R(ioTi)  and  —  £R(oil2)  are  common.  The  faces 
of  the  rhombohedron  are  frequently  curved.     Compare  (Fig.  125.) 

The  cleavage  of  siderite  is  like  that  of  the  other  minerals  of  this 
group.  Its  hardness  is  3.5-4  and  density  3.85.  In  color  the  mineral 
is  sometimes  white,  but  more  frequently  it  is  some  shade  of  yellow  or 
brown.     Its  streak  is  white.    Most  specimens  are  translucent. 


220  DESCRIPTIVE  MINERALOGY 

In  the  closed  tube  siderite  decrepitates,  blackens  and  becomes  mag- 
netic. It  is  only  slowly  affected  by  cold  acids  but  it  effervesces  briskly 
in  hot  ones. 

Siderite  is  distinguished  from  the  other  carbonates  by  its  reaction 
for  iron. 

The  mineral  changes  on  exposure  into  limonite  and  sometimes  into 
hematite  or  even  into  magnetite. 

Synthesis. — Crystals  of  siderite  may  be  obtained  by  heating  a  solu- 
tion of  FeSC>4  with  an  excess  of  CaCC>3  at  2000. 

Occurrence  and  Origin. — The  mineral  is  often  found  accompanying, 
metallic  ores  in  veins.    It  occurs  also  as  nodules  in  certain  clays  and  in 
the  coal  measures.     In  some  cases  it  appears  to  be  a  direct  deposit  from 
solutions.    In  others  it  is  a  result  of  metasomatism  and  in  others  is  an 
ordinary  weathering  product. 

Localities. — The  crystallized  variety  is  found  at  Freiberg,  in  Saxony; 
at  Harzgerode,  in  the  Harz;  at  Alston  Moor,  and  in  Cornwall,  Eng- 
land; and  along  the  Alps,  in  Styria  and  Carinthia.  Cleavage  masses 
are  present  in  the  cryolite  from  Greenland. 

Workable  beds  of  the  ore  are  present  in  Columbia  Co.,  and  at  Rossie, 
in  St.  Lawrence  Co.,  N.  Y.;  in  the  coal  regions  of  Pennsylvania  and 
Ohio,  and  in  clay  beds  along  the  Patapsco  River,  in  Maryland.  The 
massive  or  nodular  ore  from  clay  banks  is  known  as  ironstone.  The 
impure  bedded  siderite  interstratified  with  the  coal  shales  is  known 
as  black-band  ore. 

Production. — Only  10,346  tons  of  siderite  were  produced  in  the  United 
States  during  191 2,  all  of  it  coming  from  the  bedded  deposits  in  Ohio. 
This  was  valued  at  $20,000. 

Rhodochrosite  (MnC03) 

This  mineral  sometimes  occurs  in  distinct  crystals  of  a  rose-red 
color;  but  it  is  usually  found  in  cleavable  masses,  in  a  compact  form, 
or  as  a  granular  aggregate.  Sometimes  it  is  in  incrustations.  It  is 
not  of  commercial  importance  in  North  America. 

Pure  manganese  carbonate  containing  61.7  per  cent  MnO  and  38.3 
per  cent  CO2  is  rare.  The  mineral  is  usually  impure  through  the  addi- 
tion of  the  carbonates  of  iron,  calcium,  magnesium  or  zinc. 

The  most  prominent  forms  on  crystals  of  rhodochrosite  are  R(io7i), 
—  £R(oil2),   ooP2(ii2o),  oR(oooi)  and  various  scalenohedrons. 

Its  cleavage  is  perfect  parallel  to  R.  The  mineral  is  brittle.  Its 
hardness  is  about  4  and  its  density  about  3.55.  Its  luster  is  vitreous, 
and  its  color  red,  brown,  or  yellowish  gray.     Its  streak  is  white.     When 


CARBONATES  221 

heated  it  begins  to  lose  CO2  at  about  3200;  but  its  dissociation  temper- 
ature is  63  2°. 

The  mineral  is  infusible,  but  when  heated  before  the  blowpipe  it 
decrepitates  and  changes  color.  When  treated  in  the  borax  bead  it 
gives  the  violet  color  of  manganese,  and  when  fused  with  soda  on  char- 
coal it  yields  a  bluish  green  manganate.  It  dissolves  in  hot  hydro- 
chloric acid. 

There  are  bu.t  few  minerals  resembling  pure  rhodochrosite  in  appear- 
ance. From  all  of  these,  except  the  silicate,  rhodonite  (p.  380),  it  is 
distinguished  by  its  reaction  for  manganese.  It  is  distinguished  from 
rhodonite  by  its  hardness,  its  cleavage  and  its  .effervescence  with  acids. 
The  impure  varieties  are  very  like  some  forms  of  siderite,  from  which, 
of  cqurse,  the  manganese  test  will  distinguish  it. 

Synthesis. — Small  rhombohedrons  of  rhodochrosite  have  been  pro- 
duced by  heating  a  solution  of  MnSO*  with  an  excess  of  CaCCfe  at  2000 
in  a  closed  tube. 

Occurrence  and  Origin. — Rhodochrosite  occurs  in  veins  associated 
with  ores  of  silver,  lead,  copper  and  other  manganese  ores  and  in  bedded 
deposits.  It  is  the  result  of  hydrothermal  or  contact  metamorphism, 
and  of  weathering  of  other  manganese-bearing  minerals. 

Localities. — The  mineral  is  found  at  Schemnitz,  in  Hungary;  at 
Nagyag,  in  Transylvania;  at  Glendree,  County  Clare,  Ireland,  where  it 
forms  a  bed  beneath  a  bog;  at  Washington,  Conn.,  in  a  pulverulent 
form;  at  Franklin,  N.  J.;  at  the  John  Reed  Mine,  Aliconte,  Lake  Co., 
and  at  Rico,  Colo.;  at  Butte  City,  Mont.;  at  Austin,  Nev.,  and  on 
Placentia  Bay,  Newfoundland.  The  Colorado  and  Montana  specimens 
are  well  crystallized. 

Uses. — The  mineral  is  mined  with  other  ores  of  manganese.  Occa- 
sionally it  is  employed  as  a  gem  stone. 

Smithsonite  (ZnCOs) 

Smithsonite,  or  "  dry-bone  ore,"  is  rarely  well  crystallized.  It 
appears  as  druses,  botryoidal  and  stalactitic  masses,  as  granular  aggre- 
gates and  as  a  friable  earth. 

In  ZnC03  there  are  64.8  per  cent  ZnO  and  35.2  per  cent  CO2.  Smith- 
sonite usually  contains  iron  and  manganese  carbonates,  often  small 
quantities  of  calcium  and  magnesium  carbonates  and  sometimes  traces 
of  cadmium.    A  specimen  from  Marion,  Arkansas,  gave: 


ZnO 

CdO    FeO 

CaO 

CuO 

C02 

CdS 

Si02 

Total 

64.12 

.63       .14 

.38 

tr. 

34.68 

•25 

.06 

100.26 

222  DESCRIPTIVE  MINERALOGY 

The  mineral  is  closely  isomorphous  with  calcite,  R(ioTi),  —  JR(oiT2), 
4R(404i),  oo  R2(ii2o),  oR(oooi)  and  R3(2i3i)  being  present  on  many 
crystals.    The  R  faces  are  rough  or  curved. 

Its  cleavage  is  parallel  to  R(ioTi).  Its  hardness  is  5  and  its  density 
about  4.4.  The  luster  of  the  mineral  is  vitreous,  its  streak  is  white  and 
its  color  white,  gray,  green  or  brown.  It  is  usually  translucent,  occa- 
sionally transparent.  When  heated  to  3000  for  one  hour  it  loses  all  of 
its  C02. 

When  heated  in  the  closed  tube  CO2  is  driven  off,  leaving  ZnO  as  a 
yellow  residue  while  hot,  changing  to  white  on  cooling.  The  mineral 
is  infusible  before  the  blowpipe.  If  a  small  fragment  be  moistened  with 
cobalt  nitrate  solution  and  heated  in  the  oxidizing  flame  it  becomes 
green  on  cooling.  When  heated  on  charcoal  a  dense  white  vapor  is 
produced.  This  forms  a  yellow  coating  on  the  coal,  which,  when  it 
cools,  turns  white.  If  this  be  moistened  with  cobalt  nitrate  and  reheated 
in  the  oxidizing  flame  it  is  colored  green. 

The  above  reactions  for  zinc,  together  with  the  effervescence  of  the 
mineral  in  hot  hydrochloric  acid  distinguish  smithsonite  from  all  other 
compounds. 

Smithsonite  forms  pseudomorphs  after  sphalerite  and  calcite  and  is 
pseudomorphed  by  quartz,  limonite,  calamine  and  goethite. 

Synthesis. — Microscopic  crystals  of  smithsonite  may  be  produced  by 
precipitating  a  zinc  sulphate  solution  with  potassium  bicarbonate  and 
allowing  the  mixture  to  stand  for  some  time. 

Occurrence. — Smithsonite  occurs  in  beds  and  veins  in  limestones, 
where  it  is  associated  with  galena  and  sphalerite  and  usually  with  cala- 
mine (p.  396).  It  is  especially  common  in  the  upper,  oxidized  zone  of 
veins  of  zinc  ores  and  as  a  residual  deposit  covering  the  surface  of  weath- 
ered limestone  containing  zinc  minerals. 

Localities. — The  mineral  is  found  at  Nerchinsk,  Siberia;  Bleiberg, 
in  Carinthia;  Altenberg,  Aachen;  Province  of  Santander,  Spain;  at 
Alston  Moor  and  other  places  in  England;  at  Donegal,  in  Ireland;  at 
Lancaster,  Penn.;  at  Dubuque,  Iowa;  in  Lawrence  and  Marion  Coun- 
ties, Arkansas;  and  in  the  lead  districts  of  Wisconsin  and  Missouri  (see 
galena  and  sphalerite). 

The  Wisconsin  and  Missouri  localities  are  the  most  important  ones 
in  North  America.  Here  the  ore  occurs  in  botryoidal,  in  stalactitic 
and  in  earthy,  compact,  cavernous  masses  of  a  dull  yellow  color  incrusted 
with  druses  of  smithsonite  crystals,  of  calamine  and  of  other  minerals, 
principally  of  lead.    This  is  the  variety  known  as  "  dry  bone." 

Uses. — The  mineral  was  formerly  an  important  ore  of  zinc,  being " 


CARBONATES  223 

mined  alone  for  smelting.  It  is  now  mined  only  in  connection  with 
calamine  and  other  zinc  ores,  and  all  are  worked  up  together.  A  trans- 
lucent green  or  greenish  blue  variety  occurring  at  Laurium,  Greece, 
and  at  Kelly,  New  Mexico,"  is  sometimes  employed  for  ornamental  pur- 
poses. About  $650  worth  of  the  material  from  New  Mexico  was  utilized 
as  gem  material  in  191 2. 

ARAGONITE  DIVISION 

This  division  of  the  carbonates  includes  the  orthorhombic  (rhombic 
bipyramidal)  dimorphs  of  the  members  of  the  calcite  group  which, 
together,  form  a  well  characterized  isodimorphous  group.  The  carbon- 
ate of  calcium  is  found  well  crystallized  in  both  divisions,  but  the  other 
carbonates  are  common  to  one  only.  They  actually  occur  in  both  divi- 
sions, but  they  are  found  as  common  members  of  one  and  only  as 
isomorphous  mixtures  with  other  more  common  forms  in  the  other. 
Thus,  barium  carbonate  is  a  common  orthorhombic  mineral  under  the 
name  of  uitherite.  It  occurs  also  with  CaCC>3  hi  mixed  crystals  under 
the  name  baricalcite,  or  neotypey  which  is  hexagonal.  (See  also  p.  212 
and  p.  213.) 

The  common  members  of  the  aragonite  division  are: 

Aragonite  CaCC>3  Sp.  Gr.  =  2.936  a  :  b  :  c=  .6228  :  1 

Stroniianite  SrC03  =  3 .  706  =  .  6090  :  1 

Witherite  BaC(>3  =4.325  =  .  5949  :  1 

Cerussite  PbC03  =6.574  =.6102:1 


.7204 
.7266 

•7413 
.7232 


Aragonite  (CaCOs) 


Aragonite  occurs  in  a  great  variety  of  forms.  Sometimes  it  is  in 
distinct  crystals,  but  more  frequently  it  is  in  oolitic  globular  and  reni- 
form  masses,  in  divergent  bundles  of  fibers  or  of  needle-like  forms,  in 
stalactites  and  in  crusts. 

In  composition  aragonite  is  like  calcite.  It  often  contains  small 
quantities  of  the  carbonates  of  strontium,  lead  or  zinc. 

Crystals  are  often  acicular  with  steep  domes  predominating.  Some 
of  the  simplest  crystals  consist  of  ooP(no),  00P06  (010),  fPoo  (032), 
P06  (on),  4P(44i),  9P(99i)  and  ooP2(i2o)  (Fig.  116).  Twinning  is 
common.  The  twinning  plane  is  often  ooP(no).  By  repetition  this 
gives  rise  to  pseudohexagonal  forms,  resembling  an  hexagonal  prism  and 
the  basal  plane  (see  Figs.  117  and  118).    The  angle  noAiTo=63°  48'. 

The  cleavage  of  aragonite  is  distinct  parallel  to  00  P  06  (010)  and 
indistinct  parallel  to  00  P(no).  Its  hardness  is  3.5-4  and  density  about 
2.93.    Its  luster  is  vitreous  and  its  color  white,  often  tinged  with  gray, 


DESCRIPTIVE  MINERALOGY 


green  or  some  other  light  shade.  Its  streak  is  white  and  the  mineral  is 
transparent  or  translucent.  Its  indices  of  refraction  for  yellow  light  are: 
a=  1.5300,  7=  1.6857.     At  4000  it  passes  over  into  calcite. 

Before  the  blowpipe  aragonite  whitens  and  falls  to  pieces.     Other- 
wise its  reactions  are  like  those  of  calcite,  from  which  it  can  be  distio- 


^P\ 


kj>^ 


/  1  1  u  1.  in  a 

u    m   »    1   i  m  B 


Fie.  116.— Aragonite  Crystal  with  =oP,iio(m);    »Pw,  010  (S)  and  P«  ,  on  {*). 
Fig.  117. — Aragonite  Twin  and  Trilling  Twinned  about  »  P  (no). 


Fie.  118.— Trilling  of  Aragonite  Twinned  about  »P  (no).     (/I)  Cross-section. 
(B)    Resulting   pseudohexagonal   group,   resembling   an    hexagonal   prism    and 

guished  by  its  crystallization,  its  lack  of  rhombohedral  cleavage  and  its 
density. 

Synthesis. — Solutions  of  OCO3  in  dilute  H2CO3  form  crystals  of 
aragonite  when  evaporated  at  a  temperature  of  about  900.  In  general, 
hot  solutions  of  the  carbonate  deposit  aragonite,  while  cold  solutions 
deposit  calcite.  If  the  solution  contains  some  sulphate  or  traces  of 
strontium  or  lead  carbonates,  mixed  crystals  consisting  principally  of 
the  aragonite  molecule  are  formed  at  ordinary  temperature. 

Occurrence  and  Origin. — Aragonite  occurs  in  beds,  usually  with 
gypsum.    It  is  also  deposited  from  hot  waters  and  from  cold  waters 


CARBONATES  225 

containing  a  sulphate  (as  from  sea  water).  The  pearly  layer  of  oyster 
shells  and  the  body  of  the  shells  of  some  other  mollusca  are  composed 
of  calcium  carbonate  crystallizing  like  aragonite.  Aragonite  is  often 
changed  by  paramorphism  into  calcite,  pseudomorphs  of  which  after 
the  former  mineral  are  quite  common. 

Localities. — The  mineral  is  found  at  Aragon,  Spain;  at  Bilin,  in 
Bohemia;  in  Sicily;  at  Alston  Moor,  England;  and  at  a  number  of 
other  places  in  Europe.  It  occurs  in  groupings  of  interlacing  slender 
columns  (flos  ferrt) ,  in  the  iron  mines  of  Styria.  Stalactites  are  abundant 
at  Leadhills,  Lanarkshire,  Scotland,  and  a  silky  fibrous  variety  known  as 
satins  par,  at  Dayton,  England. 

In  the  United  States  crystallized  aragonite  occurs  at  Mine-la-Motte, 
Mo.,  and  in  the  lands  of  the  Creek  Nation,  Oklahoma.  Flos  ferri  has 
been  reported  from  the  Organ  Mts.,  New  Mexico,  and  fibrous  masses 
from  Hoboken,  N.  J.,  Lockport,  Edenville  and  other  towns  in  New  York 
and  from  Warsaw,  111. 

Strontianite  (SrC03) 

In  general  appearance  and  in  its  manner  of  occurrence  strontianite 
resembles  aragonite.  Its  crystals  are  often  acicular  in  habit  though 
repeated  twins  are  common.    The  angle  noAiTo=62°  41'. 

The  composition  of  pure  strontianite  is  SrO=7o.i,  ^2=29.9,  but 
the  mineral  usually  contains  an  admixture  of  the  barium  and  calcium 
carbonates. 

Strontianite  is  brittle,  its  hardness  is  3.5-4  and  its  density  3.7. 

Before  the  blowpipe  strontianite  swells  and  colors  the  flame  with  a 
crimson  tinge.  It  dissolves  in  hydrochloric  acid.  The  solution  im- 
parts a  crimson  color  to  the  blowpipe  flame.  When  treated  with  sul- 
phuric acid  it  yields  a  precipitate  of  SrSO*.  Its  refractive  indices  for 
yellow  light  are:  a  =1.5 199,  7=1.668.     Its  dissociation  temperature  is 

"55°. 

Aragonite,  witherite  (BaCOa)  and  strontianite  are  so  similar  in  ap- 
pearance and  in  general  properties  that  they  can  be  distinguished  from 
one  another  best  by  their  chemical  characteristics.  They  are  all  sol- 
uble in  hydrochloric  acid  and  these  solutions  impart  distinctive  colors 
to  the  blowpipe  flame  (see  p.  477). 

Syntheses. — Crystals  of  strontianite  are  obtained  by  precipitating 
a  hot  solution  of  a  strontium  salt  by  ammonium  carbonate,  and  by  cool- 
ing a  solution  of  SrC03  in  a  molten  mixture  of  NaCl  and  KC1. 

Occurrence. — Strontianite  occurs  in  veins  in  limestone  and  as  an 


226  DESCRIPTIVE  MINERALOGY 

alteration  product  of  the  sulphate  (celestite)  where  this  is  exposed  to  the 
weather.    It  is  probably  in  all  cases  a  deposit  from  water. 

Localities. — Strontianite  is  the  most  common  of  all  strontiaa-fiem- 
goupds.  It  frequently  occurs  as  the  filling  of  metallic  veins.  It  forms 
finely  developed  crystals  at  the  Wilhelmine  Mine  near  Munster,  West- 
phalia. At  Schoharie,  N.  Y.,  it  occurs  as  crystals  and  as  granular  masses 
in  nests  in  limestone.  It  is  found  also  at  other  places  in  New  York,  in 
Mifflin  Co.,  Penn.,  and  on  Mt.  Bannell  near  Austin,  Texas. 

Uses. — Strontium  compounds  are  little  used  in  the  arts.  The 
hydroxide  is  employed  to  some  extent  in  refining  beet  sugar  and  the 
nitrate  in  the  manufacture  of  "  red  fire."  Other  compounds  are  used 
in  medicine.  All  the  strontium  salts  used  in  the  United  States  are 
imported. 

Witherite  (BaC03) 

Witherite  differs  very  little  in  appearance  or  in  manner  of  occurrence 
from  aragonite.     Its  crystals  are  nearly  always  in  repeated  twins  that 

have  the  habit  of  hexagonal  pyramids'  (Fig. 
119).    The  angle  1 10  A 1  To  =  62°  46'. 

When  pure  the  mineral  contains  77.7  per 
cent  BaO  and  22.3  per  cent  CO2. 

It  is  much  heavier  than  the  calcium  car- 
bonate, its  density  being  4.3.     Its  hardness 
Fig.  119.— Witherite  Twinned     is   3  to  4.     Its    refractive    index  for  yellow 
about  cop  (1 10),  thus Imi-    Hghtj  /3=Ii740.     its  dissociation  tempera- 

tating  Hexagonal  Combina-  .  0 

tions.  m°° 

It  dissolves  readily  in  dilute  hydrochloric 

acid  with  effervescence,  and  from  this  solution,  even  when  dilute,  sul- 
phuric acid  precipitates  a  heavy  white  precipitate  of  BaS04,  which, 
when  heated  in  the  blowpipe  flame,  imparts  to  it  a  yellowish  green 
color. 

Witherite  is  distinguished  from  the  other  carbonates  by  its  crys- 
tallization, and  the  color  it  imparts  to  the  blowpipe  flame. 

Syntheses. — Crystals  are  produced  by  precipitating  a  hot  solution  of 
a  barium  salt  with  ammonium  carbonate,  and  by  cooling  a  molten 
magma  composed  of  NaCl  and  BaC03. 

Localities. — Witherite  is  not  a  very  common  mineral  in  the  United 
States,  but  it  occurs  in  large  quantity  associated  with  lead  minerals  in 
veins  at  Alston  Moor,  in  Cumberland  and  near  Hexham,  in  Northum- 
berland, England.  Some  of  the  crystals  found  in  these  places  measure 
as  much  as  six  inches  in  length. 


CARBONATES 


227 


Its  best  known  locality  in  the  United  States  is  Lexington,  Kentucky, 
where  the  mineral  is  associated  with  the  sulphate,  barite. 

Uses. — It  is  used  to  some  extent  as  a  source  of  barium  compounds. 
The  importations  of  the  mineral  during  1912  aggregate  $25,715. 

Cerussite  (PbC03) 

Cemssite  generally  occurs  in  crystals  and  in  granular,  earthy  and 
fibrous  masses  of  a  white  color. 

The  pure  lead  carbonate  contains  COa=i6.5  and  PbO=83.5,  bui 
the  mineral  usually  contains  in  addition  some  ZnCOs. 


Mi  -PS, «©<»); 
<*);   JP»,  onto  and 


—Cerussite  Trilling  Twinned  about  °°P(uo). 
—Cerussite  Trilling  Twinned  about  «  P3(l3o). 


Its  simple  crystals  are  tabular  combinations  of  °oP(no},  00  Pd6  (010) 
ooPob(roo)  and  various  brachydomes  (Fig.  120),  and  these  are  often 
twinned  in  such  a  way  as  to  produce  six  rayed  stars  (Fig.  121),  or  other 
symmetrical  forms  (Fig.  122).  Groups  of  interpenetrating  crystals 
are  also  common.    The  angle  iioaii°=o2°  46'. 

The  color  of  the  mineral  is  usually  white,  but  its  surface  is  frequently 
discolored  by  dark  decomposition  products.  Its  luster  is  adamantine 
or  vitreous  and  its  hardness  is  3-3.5.  Itsdensity=6.s.  Its  refractive 
indices  for  yellow  light  are:  0  =  1.8037,^=2.0763,7=2.0780. 

The  mineral  is  dissolved  by  nitric  acid  with  effervescence  and  by 
potassium  hydroxide.  Before  the  blowpipe  it  decrepitates,  turns  yellow 
and  changes  to  lead  oxide.  On  charcoal  it  is  reduced  to  a  metallic 
globule,  and  yields  a  white  and  yellow  coating. 


228  DESCRIPTIVE  MINERALOGY 

Cerussite  is  not  easily  confused  with  other  minerals.  It  is  well  char- 
acterized by  its  high  specific  gravity,  its  reaction  (or  lead,  and  is  dis- 
tinguished from  the  sulphate  (anglestie)  by  effervescence  with  hot  acids. 

Syntheses. — Crystals  have  been  obtained  by  heating  lead  formate  with 
water  in  a  closed  tube,  and  by  treatment  of  a  lead  salt  by  a  solution  of 
ammonium  carbonate  at  a  temperature  of  tso°-i8o°. 

Occurrence  and  Origin. — The  mineral  occurs  at  all  localities  at  which 
other  lead  compounds  are  found,  since  it  is  often  produced  from  thes? 


Fig.  123. — Radiate  Groups  of  Cerussite  on  Galena  from  Part  City  District,  Utah. 
(After  J.  M.  BoutmB.) 

latter  by  the  action  of  the  atmosphere  and  atmospheric  water.  It  is, 
therefore,  usually  found  in  the  upper  portions  of  veins. 

Localities. — Cerussite  crystals  of  great  beauty  are  found  in  many  of 
the  lead-producing  districts  of  Europe  and  also  at  Phoenixville,  Penn.; 
near  Union  Bridge,  in  Maryland;  at  Austin's  Mines,  Wythe  Co.,  Vir- 
ginia, and  occasionally  in  the  lead  mines  of  Wisconsin  and  Missouri. 
In  the  West  it  occurs  at  Leadville,  Colo.;  at  the  Flagstaff  and  other 
mines  in  Utah  (Fig.  123),  and  at  several  different  mines  in  Arizona. 

Uses.— It  is  mined  with  other  lead  compounds  as  an  ore  of  the  metal. 


CARBONATES  229 

Dolomite  (MgCa(C03)2) 

Dolomite  is  apparently  isomorphous  with  calcite  but  the  etch 
figures  on  rhombohedral  faces  prove  it  to  belong  in  the  trigonal 
rhombohedral  class.  It  occurs  as  crystals  and  in  all  the  forms  charac- 
teristic of  calcite  except  the  fibrous. 

Nearly  all  calcite  contains  more  or  less  magnesium  carbonate,  but 
most  of  the  mixtures  are  isomorphous  with  calcite  and  magnesite. 
When  the  ratio  between  the  two  carbonates  reaches  54.35  per  cent 
CaC03  :  45.65  per  cent  MgCC>3,  which  is  equal  to  the  ratio  between 
the  molecular  weights  of  the  two  substances,  or  in  other  words  when  the 
two  carbonates  are  present  in  the  compound  in  the  ratio  of  one  molecule 
to  one  molecule,  the  mineral  is  called  dolomite.  The  calculated  com- 
position of  dolomite  (MgCa(COs)2)  is  30.4  per  cent  CaO;  21.7  per  cent 
MgO  and  47.8  per  cent  CO2. 

The  crystals  of  dolomite  are  usually  rhombohedral  combinations  of 
the  rhombohedron  R(ioTi)  with  the  scalenohedron 
R3(2i3i)  (Fig.  124),  and  its  tetartohedral  forms, 
and  often  the  prism  ooP2(ii2o)  and  the  basal 
plane.  Its  axial  ratio  is  a  :  c=i  :  .8322.  Twins 
are  not  rare,  with  oR(oooi)  and  R(ioTi)  the 
twinning  planes.  The  R  planes  are  often  curved, 
frequently  with  concave  surfaces  (Fig.  125).  The 
angle  10T1 A  1101  =  73°.  FiG  l2  _Dolomite 

The  cleavage   of  dolomite   is  perfect   parallel         crystal  with°   R 
to   R.    The   mineral    is   brittle.     Its  hardness  is        40^I  (^  an(j  0p' 
3.5-4  and  density  2.915.     Its  luster  is  vitreous  or        0001  (c). 
pearly  and  its  color   white,   red,  green,   gray  or 
brown.     Its  streak  is  always  white  and  the  mineral  is  translucent  or 
transparent.    Its  refractive  indices   for  yellow  light  are:    a>=i.68i7, 
e=  1.5026.    The  important  varieties  recognized  are: 

Pearlspar,  with  curved  faces  having  a  pearly  luster. 

Granular  or  saccharoidal,  including  many  marbles  and  magnesian 
limestones. 

Dolomitic  limestone,  including  much  hydraulic  limestone. 

Many  dolomites  are  intermixed  with  the  carbonates  of  iron,  manga- 
nese, cobalt  or  zinc  and  these  are  known  as  ferriferous  dolomite,  etc. 

Dolomite  behaves  like  calcite  before  the  blowpipe  and  in  the  closed 
tube.  It,  however,  dissolves  only  slowly,  if  at  all,  in  cold  hydrochloric 
acid,  except  when  very  finely  powdered,  though  dissolving  readily  with 
effervescence  in  hot  acid. 


230  DESCRIPTIVE  MINERALOGY 

The  reaction  toward  cold  acid  and  its  greater  hardness  easily  dis- 
tinguish dolomite  from  catcUe.  It  is  distinguished  from  magnetite  by 
the  name  reaction. 

Occurrence  and  Origin. — Dolomite,  like  the  calcium  carbonate,  occurs 
crystallized  in  veins,  and  as  granular  masses  forming  great  beds  of  rock. 
It  is  a  precipitate  from  solutions  and  a  metasomatic  alteration  product 
of  calcite. 

Localities. — Its  crystals  are  present  at  many  places,  among  them 
Bex,  in  Switzerland;  Traversella,  in  Piedmont;  Guanajuato,  in  Mexico; 
Roxbury,  in  Vermont;  Hoboken,  N.  J.;  Niagara  Falls,  the  Quarantine 


Fig.  135. — Group  of  Dolomite  Crystals  from  Joplin,  Mo.    Flat  Rhombobcdrons  with 
Curved  Faces. 

Station,  and  Putnam,  N.  V.;  Joplin,  Mo.;  and  Stony  Point,  N.  C.    It 
is  also  very  widely  spread  as  beds  of  dolomitic  limestone. 

Uses. — Dolomite  is  used  for  many  of  the  purposes  served  by  calcite; 
indeed,  much  of  the  material  used  as  marble,  limestone,  etc.,  contains  a 
large  percentage  of  magnesium  carbonate.  It  is  not,  however,  used  as  a 
flux  or  in  the  manufacture  of  Portland  cement,  nor  as  a  source  of  lime. 

Ankerite(Ca(Mg-Fe)(C03)2)  is  a  ferruginous  dolomite.  It  is  an 
isomorphous  mixture  of  the  carbonates  of  calcium,  magnesium  and  iron, 
in  which  the  FeC03  replaces  a  part  of  the  MgCOs  in  dolomite.  It  is 
usually  in  rhombohedral  crystals,  with  the  angle  10T1  aTioi  =  73°  48'. 
Its  color  is  white,  gray  or  red  and  its  streak  is  white.  Its  hardness 
=3.5-4,  and  its  density  =  2.98.  It  also  occurs  in  coarse  and  fine-grained 
granular  masses.    Ankerite  is  infusible  before  the  blowpipe.     In  the 


CARBONATES  231 

closed  tube  it  darkens  and  when  heated  on  charcoal  it  becomes  mag- 
netic. It  occurs  in  veins,  especially  those  containing  iron  minerals. 
It  has  been  found  at  Antwerp  and  other  places  in  northern  New  York. 

CALCIUM-BARIUM  CARBONATES 

Carbonates  of  the  general  composition  CaBa(COs)2  occur  (i)  as  a 
series  of  mixed  crystals  isomorphous  with  calcite  under  the  name  bari- 
calcite;  (2)  as  a  series  of  mixed  crystals  isomorphous  with  aragonite 
known  as  alstonite  or  bromlite,  and  (3)  a  typical  double  salt,  barytocalcite, 
which  is  monoclinic.  Both  alstonite  and  barytocalcite  occur  in  veins 
of  lead  ores  and  of  barite  (BaSC>4). 

Barytocalcite,  CaBa(COs)2  is  monoclinic  (prismatic  class),  with 
a  :  b  :  £=.7717  :  1  :  6255  and  $  =  73°  52'.  It  forms  crystals  bounded 
by  00  P  6b  (100),  ooP(no),  oP(ooi),  and  a  series  of  clinopyramids,  of 
which  2P2  (1 2!)  and  5P5  (1 5!)  are  common.  It  also  occurs  massive.  Its 
perfect  cleavage  is  parallel  to  ooP(no).  The  mineral  is  white,  gray, 
greenish  or  yellowish.  Its  streak  is  white,  hardness =4  and  sp.  gr.= 
3.665.  It  is  transparent  or  translucent.  Before  the  blowpipe  frag- 
ments fuse  on  thin  edges,  and  assume  a  pale  green  color,  due  to  the 
presence  of  a  little  manganese.  The  mineral  is  soluble  in  HC1.  Its 
principal  occurrence  is  Alston  Moor,  Cumberland,  England. 

BASIC  CARBONATES 

The  basic  carbonates  are  salts  in  which  all  or  a  portion  of  the  hydro- 
gen of  carbonic  acid  is  replaced  by  the  hydroxides  of  metals.  There 
are  only  three  minerals  belonging  to  the  group  that  need  be  referred  to 
here.  Two  are  copper  compounds.  One  is  the  bright  green  malachite 
and  the  other  the  blue  azurite.    The  composition  of  the  former  may  be 

CuOHv 
represented  by  the  formula  7CO3,  and  that  of  the  latter  by 

CuOH' 
CuOHv 

Cu=(COs)2.  Both  are  used  to  some  extent  as  ores  of  the  metal, 
CuOH/ 

though  their  value  for  this  purpose  is  not  great  at  the  present  time. 
They  may  easily  be  distinguished  from  all  other  minerals  by  their 
distinctive  colors,  by  the  fact  that  they  yield  water  in  the  closed  tube 
and  by  their  effervescence  with  acids.  The  third  mineral  (hydrozincite) 
is  a  white  substance  that  occurs  as  earthy  or  fibrous  incrustations  on  other 
zinc  compounds.    Its  composition  corresponds  to  2ZnC03*3Zn(OH)2. 


232 


DESCRIPTIVE  MINERALOGY 


Its  hardness  =  2-2.5  an^  *ts  specific  gravity  is  about  3.7.    Only  the  two 
copper  compounds  are  described  in  detail. 


Malachite  ((CuOH)2C03) 

Malachite  usually  occurs  in  fibrous,  radiate,  stalactitic,  granular 
or  earthy,  green  masses,  or  as  small  drusy  crystals  covering  other  copper 
compounds.  The  mineral  contains,  when  pure,  19.9  per  cent  CO2, 
71.9  per  cent  CuO  and  8.2  per  cent  H2O. 

Well  defined  crystals  are  usually  very  small  monoclinic  prisms  (mon- 

oclinic  prismatic  class),  with  an  axial  ratio  .8809  :  1 
:  .4012  and  0=6i°  50'.  Their  predominant  forms 
are  00  Pco  (100),  00  Pod  (010),  ooP(no),  and 
oP(ooi).  Contact  twins  are  common,  with 
00  Pco  (100)  the  twinning  plane  (Fig.  126).  The 
angle  no  A  1^0=75°  40'. 

The  pure  mineral  is  bright  green  in  color  and  has 
a  light  green  streak.  It  possesses  a  vitreous  luster, 
but  this  becomes  silky  in  fibrous  masses  and  dull 
in  massive  specimens.  Crystals  are  translucent 
and    massive    pieces    are    opaque.      Translucent 


Fig.  126. — Malachite 
Crystal  with  00  P, 
no   (m);      00 Poo, 

100   (o),  and    oP,   pieces  are  pleochroic   in  yellowish  green  and  dark 
001  (c)  Twinned   green  tmts     fne  deavage  is  perfect  parallel   to 

oP(ooi).  The  hardness  of  malachite  is  3.5-4,  and 
its  density  about  3.9.     Its  refractive  index,  /?,  for  yellow  light=i.88. 

Malachite  turns  black  and  fuses  before  the  blowpipe  and  tinges  the 
flame  green.  With  Na2C03  on  charcoal  it  yields  a  copper  globule.  It  is 
difficultly  soluble  in  pure  water,  but  is  easily  dissolved  in  water  con- 
taining CO2.  It  is  soluble  with  effervescence  in  HC1  and  its  solution 
becomes  deep  blue  on  the  addition  of  an  excess  of  ammonia.  When 
heated  in  a  closed  glass  tube,  it  gives  an  abundance  of  water.  Boiled 
with  water  it  turns  black  and  loses  its  CO2. 

Malachite,  on  account  of  its  characteristic  color,  may  be  easily  dis- 
tinguished from  all  other  minerals  but  some  varieties  of  turquoise  and 
a  few  copper  compounds,  such  as  atacamile  (p.  144).  It  may  be  dis- 
tinguished from  all  of  these  by  its  effervescence  with  acids. 

Synthesis. — Malachite  crystals  have  been  obtained  with  the  form  of 
natural  crystals  by  heating  a  solution  of  copper  carbonate  in  ammonium 
carbonate. 

Occurrence  and  Origin. — Malachite  is  a  frequent  decomposition 
product  of  other  copper  minerals,  being  formed  rapidly  in  moist  places. 


CARBONATES  233 

It  occurs  abundantly  in  the  upper  oxidized  portions  of  veins  of  copper 
ore,  where  it  is  associated  with  azurite,  cuprite,  copper,  limonite  and  the 
sulphides  of  iron  and  copper,  often  pseudomorphing  the  copper  minerals. 
The  green  stain  noticed  on  exposed  copper  trimmings  of  buildings  is 
composed  in  part  of  this  substance. 

Localities. — The  mineral  occurs  in  all  copper  mines.  At  Chessy, 
France,  it  forms  handsome  pseudomorphs  after  cuprite.  In  the  United 
States  it  has  been  found  in  good  specimens  at  Cornwall,  Lebanon  Co., 
Penn.;  at  Mineral  Point,  Wisconsin;  at  the  Copper  Queen  Mine,  Bisbee, 
and  at  the  Humming  Bird  Mine,  Morenci,  Arizona,  and  in  the  Tintic 
district,  Utah. 

Uses. — In  addition  to  its  use  as  an  ore  of  copper  the  radial  and  mass- 
ive forms  of  malachite  are  employed  as  ornamental  stones  for  inside 
decoration.  The  massive  forms  are  also  sawn  into  slabs  and  polished 
for  use  as  table  tops  and  are  turned  into  vases,  etc. 

Production. — As  malachite  is  mined  with  other  copper  compounds, 
the  quantity  utilized  as  an  ore  of  the  metal  is  not  known.  The  amount 
produced  in  the  United  States  during  191 2  for  ornamental  purposes  was 
valued  at  $1,085.  This,  however,  included  also  a  mixture  of  malachite 
and  azurite. 

Azurite  (Cu(CuOH)2(C03)2) 

Azurite  is  more  often  found  in  crystals  than  is  malachite.  It  occurs 
also  as  veins  and  incrustations  and  in  massive,  radiated,  and  earthy 


Fig.  127. — Azurite  Crystals  with  oP,  001  (c);     — Poo,  101  (<r);     00  Poo,  100  (a); 
P,  In  (*),  00 P,  no  (w);   -2P,  221  (A);  JP2,  243  (d)  and  Poo  ,  on  (/). 

forms  associated  with  malachite  and  other  copper  compounds.     Its 
most  frequent  associate  is  malachite,  into  which  it  readily  alters. 

In  composition  azurite  is  25.6  per  cent  CO2,  69.2  per  cent  CuO,  and 
5.2  per  cent  H2O.  It  changes  rapidly  to  malachite,  and  sometimes  is 
reduced  to  copper. 

The  crystals  are  tabular,  prismatic,  or  wedge-shaped  monoclinic 
forms  (monoclinic  prismatic  class),  with  an  axial  ratio  a  :  b  :  £=.8501  : 
1  :  1.7611,  and  £=87°  36'.    They  are  usually  highly  modified,  58  or 


234  DESCRIPTIVE  MINERALOGY 

more  different  planes  having  been  identified  on  them.  The  predominant 
ones  are  oP(ooi),  — P<»(ioi),  oo  P(no),  —  2P(22i)  and  ooPao(ioo). 
(Fig.  127.)     The  angle  noAiTo=8o°  40'. 

The  mineral  is  dark  blue,  vitreous,  and  translucent  or  transparent, 
and  is  pleochroic  in  shades  of  blue.  It  is  brittle.  Its  streak  is  light 
blue,  its  hardness  3.5-4  and  density  3.8.  Its  cleavage  is  distinct  parallel 
to  Pob  (on). 

The  blowpipe  and  chemical  reactions  for  azurite  are  the  same  as 
those  for  malachite.  By  them  the  mineral  is  easily  distinguished  from 
the  few  other  blue  minerals  known. 

Synthesis. — Crystals  have  been  formed  on  calcite  by  allowing  frag- 
ments of  this  mineral  to  lie  in  a  solution  of  CUNO3  for  a  year  or  more. 

Occurrence. — The  mineral  occurs  in  the  oxidized  zone  of  copper  veins. 
It  is  an  intermediate  product  in  the  change  of  other  copper  compounds 
to  malachite. 

Localities. — Azurite  occurs  in  beautiful  crystals  at  Cressy,  France; 
near  Redruth,  in  Cornwall;  at  Phoenixville,  Penn.;  at  Mineral  Point, 
Wis.;  at  the  Copper  Queen  Mine,  Bisbee,  Ariz.;  at  the  Mammoth 
Mine,  Tintic  district,  Utah;  at  Hughes's  Mine,  California,  and  at  many 
other  copper  mines  in  this  country  and  abroad. 

From  Morenci,  Ariz.,  Mr.  Kunz  describes  specimens  consisting  of 
spherical  masses  composed  of  alternating  layers  of  malachite  and 
azurite,  which,  when  cut  across,  yield  surfaces  banded  by  alternations  of 
bright  and  dark  blue  colors. 

Uses. — Azurite  is  mined  with  other  copper  minerals  as  an  ore  of  cop- 
per. It  is  also  used  to  a  slight  extent  as  an  ornamental  stone  (see  mal- 
achite). 

HYDROUS  CARBONATES 

The  hydrous  carbonates  are  salts  containing  water  of  crystalliza- 
tion. They  are  carbonates  of  sodium  or  of  this  metal  with  calcium  or 
magnesium.  Some  of  them  occur  in  abundance  in  the  waters  of  salt  or 
bitter  lakes,  but  very  few  are  known  to  occur  in  any  large  quantity  in 
solid  form.    Among  the  commonest  are: 

Soda  or  natron  Na2C03  •  10H2O  monoclinic 

Trona  HNa3(C03)2-2H20  monoclinic 

Gaylussite  Na2Ca(C03)2  •  5H2O  monoclinic 

HydromagnesUe  Mg4(OH)2(C03)3  •  3H2O  orthorhombic 

These  minerals  occur  either  in  the  muds  of  lakes  or  as  crusts  upon  the 
mud  or  upon  other  minerals. 


CARBONATES  235 

Natron  occurs  only  in  solution  and  in  the  dry  mud  on  the  borders 
of  lakes. 

Trona,  or  urao,  (HNa3(C03)2  2H2O)  is  found  as  crystals  in  the 
mud  of  Borax  Lake,  California,  as  a  massive  bed  in  Churchill  Co., 
Nevada,  and  as  thin  coatings  on  rocks  in  other 
places.  Its  crystallization  is  monoclinic  (pris- 
matic class),  with  the  axial  ratio,  2.8426  :  1  : 
2.9494  and  0=76°  31'.  Its  crystals  are  usually 
bounded  by  oP(ooi),  00  Poo  (ioo),  — P(in)  and  Fig.  128.— Trona  Crys- 
+P(Tn)  (Fig.  128).  Fibrous  and  massive  forms  tal  with  oP,ooi  (c); 
are  common.  The  mineral  has  a  perfect  cleavage  °° p  * '  IO°  ^  and 
parallel  to  00  Pdb  (100).    It  is  gray  or  yellowish  \11 

and  has  a  colorless  streak.  It  has  a  vitreous  luster,  a  hardness  of 
2.5-3,  and  a  density  of  2.14.  It  is  soluble  in  water  and  has  an  alkaline 
taste.    It  exhibits  the  usual  reactions  for  Na  and  for  carbonates. 

Gaylussite  (Na2Ca(C03)2-5H20)  also  occurs  as  crystals  in  the 
muds  of  certain  lakes,  especially  Soda  Lake,  near  Ragtown,  Nevada, 
and  Merida  Lake,  Venezuela,  and  in  clays  under  swamps  in  Railroad 

Valley,  in  Nevada.  Its  crystals  are  monoclinic 
(prismatic  class)  with  alb:  c— 1.4897  :  1  :  1.4442 
and  18=78°  27'.  They  are  usually  bounded  by 
00  P(no),  Po>  (on),  and  £P(Ti2)  (Fig.  129),  or  by 
these  planes  and  oP(ooi)  and  00  P  60  (100).  They 
are  either  prismatic  because  of  the  predominance 
of  Poo  (on)  and  oP(ooi),  or  are  octahedral  in 
habit  because  of  the  nearly  equal  development  of 
P  00  (on)  and  00  P(no).  Their  cleavage  is  perfect 
Fig.  1 29.- Gaylussite    parallel  to  ooP(no). 

t\  W^        '         The  mineral  is  white  or  yellowish  and  trans- 
no  (w);  Poo  ,  on  J 

(e)  and  iP,  112  (r).    lucent.      Its   hardness   is    2-3    and    density    1.99. 

It  is  very  brittle.  When  heated  in  the  closed 
tube  it  decrepitates  and  becomes  opaque.  It  loses  its  water  at  ioo0. 
In  the  flame  it  melts  easily  to  a  white  enamel  and  colors  the  flame  yellow. 
It  is  partially  soluble  in  water,  leaving  a  white  powdery  residue  of  CaCCfe 
and  is  entirely  soluble  in  acids  with  effervescence.  The  mineral  occurs 
in  such  large  quantity  in  the  clays  underlying  swamps  in  Railroad  Valley, 
Nevada,  that  its  use  has  been  suggested  as  a  source  of  NagCOs. 


CHAPTER  XII 

THE   SULPHATES 

The  sulphates  are  salts  of  sulphuric  acid.  A  large  number  are 
known  to  occur  in  nature  but  many  of  them  are  dissolved  in  the  waters 
of  salt  lakes.  Of  the  remaining  ones  only  a  few  are  very  common. 
These  may  be  divided  into  an  anhydrous  normal  group,  a  basic  group  and 
a  hydrated  group.  In  addition,  there  are  several  minerals  that  are 
sulphates  mixed  with  chlorides  or  carbonates. 

All  the  sulphates  that  are  soluble  in  water  give  the  test  for  sulphuric 
acid.  When  heated  with  soda  on  charcoal  they  are  reduced  to  sulphides. 
The  mass  when  placed  on  a  silver  coin  and  moistened  with  a  drop  of 
water  or  of  hydrochloric  acid  partly  dissolves  and  stains  the  silver  dark 
brown  or  black. 

The  sulphates  when  pure  are  all  white  and  transparent,  and  are  all 
nonconductors  of  electricity. 

ANHYDROUS  SULPHATES 

NORMAL  SULPHATES 

The  anhydrous  normal  sulphates  have  the  general  formula  R'2S04 
or  R"S04.  The  most  common  ones  are  sulphates  of  the  alkaline  earths 
and  lead.  They  belong  in  a  single  group  which  is  orthorhombic.  The 
few  less  common  ones  are  sulphates  of  the  alkalies  or  of  the  alkalies 
and  alkaline  earths.     Only  two  of  the  latter  are  described. 

Glauberite  (Na2Ca(S04)2) 

Glauberite  may  be  regarded  as  a  double  salt  of  the  composition 
Na2S04  •  CaS04,  which  requires  51.1  per  cent  Na2S04  and  48.9  per  cent 
CaS04.  The  mineral  contains  22.3  per  cent  Na20,  20.1  per  cent  CaO 
and  57.6  per  cent  SO3. 

It  nearly  always  occurs  in  monoclinic  crystals  (prismatic  class), 
with  an  axial  ratio  1.2209  :  1  :  1.0270  and  P=6j°  49'.  The  most  fre- 
quent combination  is  oP(ooi),  — P(ni),  ooP(no),  ooPoo(ioo), 
3P3(3iT)  and  +P(nT),  with  oP(ooi)  prominent  (Fig.  130).  The 
cleavage  is  perfect  parallel  to  oP(ooi).    The  angle  no  A  110=96°  58'. 

236 


SULPHATES 


237 


Glauberite  is  yellow,  gray  or  brick-red  in  color,  is  transparent  or 
translucent  and  has  a  white  streak,  a  vitreous  luster  and  a  conchoidal 
fracture.  Its  hardness  is  2.5-3  and  its  specific 
gravity  about  2.8.  It  is  brittle.  It  is  partly 
soluble  in  water,  imparting  to  the  solution  a 
slight  saltiness.  The  red  color  of  many  speci- 
mens is  due  to  the  presence  of  inclusions. 

Before  the  blowpipe  the  mineral  decrepi- 
tates, whitens    and    fuses  easily   to  a  white 
enamel,  at  the  same  time  coloring  the  flame  Fig.  130— Glauberite  Crys- 
yellow.     It  is  soluble  in  HC1  and  in  a  large      tal  with  oP,  001,  (c);  °oP, 

i.\L         £  t  11  4. 'a.         e       no  (m);  oo  P  oo ,  ioo  (a) 

quantity  of  water.    In  a  small  quantity   of         h  -P        () 
water  it  is  partially  dissolved  with   loss   of 
transparency  and  the  production  of  a  deposit  of  CaS04. 
It  sometimes  alters  to  calcite. 

Occurrence. — Glauberite  is  associated  with  rock  salt  and  other  de- 

posits  from  bodies  of  salt  water.  It  is  found 
at  Villa  Rubia,  in  Spain,  and  elsewhere 
in  Europe,  and  in  the  Rio  Verde  Valley, 
Arizona  and  at  Borax  Lake,  California. 


Fig.  131. — Thenardite  Crystal 
with  00  P,  no  (w);  P,  11T 
(0);  iP«,  106  (/)  and  oP, 
oor(c). 


Thenardite  (Na2S04)  occurs  as  ortho- 
rhombic  crystals  in  the  vicinity  of  salt 
lakes,  and  in  beds  associated  with  other 
lake  deposits.  Its  crystals  have  an  axial  ratio  .5976  :  1  :  1.2524. 
They  are  commonly  prismatic  but  those 
from  California  are  tabular  and  are  bounded 
by  ooP(no),  oP(ooi),  P(iiT),  JP *>  (106), 
and  00  P*  (100)  (Fig.  131).  Twins  arc 
common  (Fig.  132). 

The  mineral  is  colorless,  white  or  reddish 
and  has  a  salty  taste.  Its  hardness  is  2-3 
and  its  specific  gravity  2.68.  Its  inter- 
mediate refractive  index  is  1.470.  It  is 
readily  soluble  in  water.  It  occurs  in  exten- 
sive deposits  in  the  Rio  Verde  Valley,  Ari- 
zona, and  as  crystals  at  Borax  Lake,  Cali- 
fornia and  on  the  shores  of  salt  lakes  in 
Central  Asia  and  South  America. 


Fig.  132.  —  Thenardite 
Twinned  about  P  06  (on). 
Forms  same  as  in  Fig.  131 
and  00  P  «  ,  100  (a). 


238  DESCRIPTIVE  MINERALOGY 


BARITE  GROUP 

The  barite  group  includes  the  sulphates  of  the  alkaline  earths  and 
lead.  They  are  all  light  colored  minerals  with  a  nonmetallic  luster. 
They  all  crystallize  in  the  orthorhombic  system  (bipyramidal  class), 
and  all  have  a  hardness  of  about  4.  The  minerals  comprising  this  group, 
with  their  axial  ratios,  are: 

Anhydrite  CaSC>4  alb:  £=.8932  :  1  :  1.0008 

Barite         BaSC>4  =.8152  :  1  :  1.3136 

Celestite      SrSQi  =.7790  :  1  :  1.2800 

AnglesUe    PbS04  =-7852  :  1  :  1.2894 

Anhydrite  (CaSOj 

Calcium  sulphate  is  dimorphous.  The  natural  compound,  anhy- 
drite, is  orthorhombic  bipyramidal.  In  addition  to  this,  there  is  another 
which  passes  over  into  anhydrite  when  shaken  for  a  long  time  with  boiling 
water.  It  is  produced  by  dehydrating  gypsum  at  about  ioo°.  When 
moistened  it  combines  with  water  and  passes  back  to  gypsum.  It  is 
probably  triclinic.  It  is  unstable  under  the  conditions  prevailing  at 
the  earth's  surface  and  is,  therefore,  not  found  as  a  mineral. 

Anhydrite  occurs  usually  in  fibrous,  granular  or  massive  forms,  not 
often  in  crystals.  When  crystals  occur  they  are  commonly  prismatic  or 
tabular  in  habit. 

In  composition  the  mineral  is  58.8  per  cent  SO3  and  41.2  per  cent 
CaO. 

Its  crystals  are  usually  bounded  by  the  three  pinacoids  oP(ooi), 
00 Poo  (100),  00 Poo  (010)  and  P(ni),  2P2(i2i),  3P3(i3i),  P*(ioi) 
and  P06  (on).  The  prismatic  types  are  usually  elongated  parallel  to 
the  macroaxis.    The  angle  noAiTo=83°  41'. 

Anhydrite  fuses  quite  easily  before  the  blowpipe  and  colors  the  flame 
reddish  yellow.  It  is  very  slightly  soluble  in  water  but  is  completely 
dissolved  in  strong  sulphuric  acid.  It  cleaves  parallel  to  the  three  pin- 
acoids yielding  rectangular  fragments.  Its  hardness  is  3-3.5  and  den- 
sity about  2.95.  Its  luster  is  vitreous  in  massive  pieces  and  its  color 
white,  often  with  a  distinct  tinge  of  blue,  gray  or  red.  In  small  frag- 
ments it  is  translucent,  but  in  large  masses  it  is  opaque.  Its  refractive 
indices  for  yellow  light  are:  a=  1.5693,  7=  1.6 130. 

It  is  distinguished  from  the  other  sulphates  by  its  specific  gravity 
and  the  color  it  imparts  to  the  blowpipe  flame. 


SULPHATES  239 

Synthesis. — Its  crystals  have  been  produced  by  slowly  evaporating  a 
solution  of  gypsum  in  H2SO4. 

Occurrence. — Anhydrite  occurs  as  crystals  implanted  on  the  minerals 
of  ore  veins,  as  beds  of  granular  masses  associated  with  gypsum,  and  as 
crystalline  masses  in  layers  associated  with  rock  salt — the  two  having 
been  deposited  by  the  evaporation  of  salt  waters. 

Localities. — The  mineral  is  found  at  the  salt  mines  of  Stassfurt,  in 
Germany;  Hall,  in  Tyrol;  Bex,  in  Switzerland;  in  the  ore  veins  of 
Andreasberg,  in  Harz;  Bleiberg,  in  Carinthia,  and  at  many  other  places 
in  Europe.  At  Lockport,  N.  Y.,  and  at  Nashville,  Tenn.,  it  occurs  as 
crystals  lining  geodes  in  limestone,  and  at  the  mouths  of  the  Avon  and 
St.  Croix  Rivers  in  Nova  Scotia  it  forms  large  beds  associated  with 
gypsum. 

Uses. — Finely  granular  forms  of  the  mineral  are  used  for  ornamental 
purposes,  and  as  a  medium  for  the  use  of  sculptors.  The  massive  variety 
is  occasionally  employed  as  a  land  plaster  to  enrich  cultivated  soils. 

Barite  (BaS04) 

Barite,  or  heavy  spar,  usually  occurs  crystallized,  though  it  is  also 
often  found  massive  and  in  granular,  fibrous  and  lamellar  forms.  It  is 
a  common  mineral  associated  with  sulphide  ores  as  a  gangue. 

The  mineral  is  sometimes  pure  but  it  is  usually  intermixed  with  the 
isomorphous  calcium  and  strontium  sulphates.  The  pure  mineral  con- 
tains 34.3  per  cent  SO3  and  65.7  per  cent  BaO.  As  usually  mined  it 
contains  SiCfe,  CaO,  MgO,  AI2O3,  Fe2(h  and  in  some  instances  PbS2 
(galena). 

The  simple  crystals  are  usually  tabular  or  prismatic  in  habit.  The 
tabular  forms  are  commonly  bounded  by  oP(ooi),  ooP(no)  and  the 
domes,  P  66  (101),  JP  w  (102),  2P  66  (021),  and  P  66  (on),  and  sometimes 
P(ni)  and  00  Poo  (100)  (Fig.  133).  The  prismatic  forms  are  usually 
elongated  in  the  direction  of 
the  a  axis,  and  are  bounded 
by  the  same  planes  as  the 
tabular  crystals  (Fig.  134).  FlG>  I33._Barite  Crystals  with  00  P,  no  (m); 
Complex  crystals  are  also  JP5o,  102  (rf);  P£,oii  (0)  and  oP,  001  (c). 
abundant.    They  are  often 

beautifully   supplied  with  planes,  the   total   number   known   on   the 
species  being  about  100.     The  angle  noAiTo=78°  225'. 

The  cleavage  of  barite  is  perfect  parallel  to  oP(ooi)  and  ooP(no). 
It  is  brittle.     Its  hardness  is  about  3  and  its  density  about  4.5.    The; 


^r^ 


240  DESCRIPTIVE  MINERALOGY 

mineral  is  white,  often  with  a  tinge  of  yellow,  brown,  blue,  or  red. 

It   is  transparent  or  opaque  and  its  streak  is  white.    Its  refractive 

indices  for  yellow  light  are:  0=1.6369,  7=1.6491. 

Before  the  blowpipe  barite  decrepitates  and  fuses,  at  the  same  time 

coloring  the  flame  yel- 
lowish green.  The  fused 
mass  reacts  alkaline  to 
litmus  paper.  It  is  in* 
soluble  in  acids. 

The  mineral  barite  is 
Fig.  i34.-Barite  Crystals  with  m,  dt  0  and  c  as  in  distinguished   from    the 

pTg'x«3wAlso  °°P55'  IO° (fl);  P'  IXI  w  "d  other  sulPhates  by its 

high  specific  gravity  and 


the  color  it  imparts  to  the  blowpipe  flame. 

Syntheses. — Crystals  have  been  made  by  heating  precipitated  barium 
sulphate  with  dilute  HC1  in  a  closed  tube  at  150°,  and  by  cooling  a  fusion 
of  the  sulphate  in  the  chlorides  of  the  alkalies  or  alkaline  earths. 

Occurrence  and  Origin. — Barite  is  a  common  vein-stone.  It  con- 
stitutes the  gangue  of  many  ore  veins,  particularly  those  of  copper, 
lead  and  silver.  It  is  found  also  as  a  replacement  of  limestone,  which, 
when  it  weathers,  leaves  the  barite  in  the  form  of  fragments  and  nodules 
in  a  residual  clay,  and  as  a  deposit  in  hot  springs.  In  all  cases  it  is 
believed  to  be  a  deposit  from  solutions. 

Localities. — Barite  occurs  abundantly  in  England,  Scotland,  and  on 
the  continent  of  Europe.  Crystals  are  found  at  Cheshire,  Conn.;  at 
DeKalb,  St.  Lawrence  Co.,  N.  Y.j  at  the  Phoenix  Mine  in  Cabarrus 
Co.,  N.  C,  and  near  Fort  Wallace,  New  Mexico.  Massive  barite  in 
pieces  large  enough  to  warrant  polishing  is  found  on  the  bank  of 
Lake  Ontario,  at  Sacketts  Harbor,  N.  Y.  It  constitutes  the  filling  of 
veins  at  many  different  places,  more  particularly  in  the  southern  Appa- 
lachians and  in  the  Lake  Superior  region. 

Preparation. — Much  of  the  mineral  that  enters  the  trade  in  the 
United  States  is  obtained  from  the  deposits  in  residual  clay.  The  rough 
material  is  washed,  hand  picked,  crushed,  ground  and  treated  with 
sulphuric  acid.  The  acid  dissolves  most  of  the  impurities  and  leaves 
•the  powdered  mineral  white. 

Uses. — The  white  varieties  of  the  mineral  are  ground  and  the  powder 
is  used  in  making  paints.  The  mineral  is  also  employed  in  the  manu- 
facture of  paper,  oilcloth,  enameled  ware,  and  in  the  manufacture  of 
barium  salts,  the  most  important  of  which  is  the  hydroxide,  which  is 
employed  in  refining  sugar. 


SULPHATES 


241 


The  colored  massive  varieties,  more  especially  stalactitic  and  fibrous 
forms,  are  sawn  into  slabs,  polished  and  used  as  ornamental  stones. 

Production. — The  quantity  of  barite  mined  in  the  United  States 
during  1912  was  over  37,000  tons,  valued  at  $153,000.  The  principal 
producing  states  are  Missouri,  Tennessee  and  Virginia.  The  imports 
in  the  same  year  were  about  26,000  tons  of  crude  material,  valued  at 
$52,467  and  3,679  tons  of  manufactured  material,  valued  at  $26,848. 
Besides,  there  were  imported  $70,300  worth  of  artificial  barium  sul- 
phate and  about  $280,000  worth  of  other  barium  salts,  exclusive  of 
witherite. 

Celestite  (SrS04) 

Celestite  occurs  in  tabular  prismatic  crystals,  in  fibrous  and  some- 
times in  globular  masses.  Though  usually  white,  it  often  possesses  a 
bluish  tinge,  to  which  it  owes  its  name. 

The  theoretical  composition  of  the  mineral  is  43.6  per  cent  SO3 
and  56.4  per  cent  SrO,  but  it  often  contains  small  quantities  of  the 
isomorphous  Ca  and  Ba  compounds. 

Many  celestite  crystals  are  very  similar  in  habit  to  those  of  barite. 


Fig.  135. — Celestite  Crystals  with   <*>  P,  no  (m);   JP«o,  102  (d);   J  Poo,  104  (r); 

00  P  00  ,  010  (6);  P  00  ,  on  (0)  and  oP,  001  (c). 


Tabular  forms  are  perhaps  more  common  (Figs.  135).  Occasionally, 
pyramidal  crystals  are  bounded  by  P4(i44),  00 Poo  (100),  Po6(on) 
and  oP(ooi).  These  often  have  rounded  edges  and  curved  faces  and 
thus  come  to  have  a  lenticular  shape.    The  angle  no  A  iTo=  75  °  50'. 

The  cleavage  of  the  mineral  is  perfect  parallel  to  oP(ooi)  and  almost 
perfect  parallel  to  ooP(no).  Its  hardness  is  about  3  and  its  specific 
gravity  3.95.  Its  luster  and  streak  are  like  those  of  barite.  Its  color 
is  often  pale  blue  and  sometimes  light  red,  but  pure  specimens  are 
white  or  colorless.  Its  refractive  indices  for  yellow  light  are:  a=  1.6220, 
7=1.6237. 

Before  the  blowpipe  celestite  reacts  like  barite  except  that  it  tinges 
the  flame  crimson.  This  crimson  color  may  be  obtained  more  dis- 
tinctly by  fusing  a  little  powder  of  the  mineral  on  charcoal  in  the  reduc- 


242  DESCRIPTIVE  MINERALOGY 

ing  flame  and  dissolving  the  resulting  mass  in  a  small  quantity  of  hydro- 
chloric acid,  then  adding  some  alcohol  and  igniting  the  mixture. 

Syntheses. — Crystals  of  celestite  are  produced  in  ways  analogous 
to  those  in  which  barite  crystals  are  formed. 

Occurrence  and  Origin. — Celestite  occurs  in  beds  with  rock  salt  and 
gypsum,  as  at  Bex,  Switzerland;  associated  with  sulphur,  as  at  Gir- 
genti,  Italy;  and  in  crystals  and  grains  scattered  through  limestone, 
as  at  Strontian  Island,  Lake  Erie,  and  in  Mineral  Co.,  W.  Va.,  or 
as  crystals  lining  geodes  in  the  same  rock.  It  is  also  sometimes  found 
as  a  gangue  in  mineral  veins.  In  some  instances  it  was  deposited  by 
hot  waters,  in  others  by  cold  waters,  and  in  others  it  was  concentrated 
by  the  leaching  of  strontium-bearing  limestones  by  atmospheric  water. 

Production  and  Uses. — Although  the  mineral  occurs  in  large  quan- 
tity at  a  number  of  places  in  the  United  States  and  Canada  it  is  not 
mined.  A  small  quantity  of  the  strontium  oxide  is  annually  imported. 
Strontium  salts,  prepared  from  celestite  in  part,  are  used  in  the  manu- 
facture of  fireworks  and  medicines  and  in  refining  sugar. 

Anglesite  (PbS04) 

Anglesite  occurs  principally  as  crystals  associated  with  galena  and 
other  ores  of  lead,  but  is  found  also  massive,  and  in  granular,  stalactitic 
and  nodular  forms. 

The  theoretical  composition  of  the  mineral  demands  73.6  per  cent 
PbO  and  26.4  SO3. 

Its  orthorhombic  crystals  are  usually  prismatic  or  isometric  in  habit. 
Tabular  habits  are  less  common  than  in  barite  and  celestite.  The 
principal  forms  occurring  are  ooP<x>  (100),  o°P(no),  £P  *  (102),  and 
other  macrodomes,  P  06  (on)  and  various  small  pyramids,  with  oP(ooi), 
in  addition,  on  the  tabular  crystals  (Figs.  156,  137,  138).  The  angle 
no  A  iTo=76°  i6|'. 

The  cleavage  of  anglesite  is  distinct  parallel  to  oP(ooi)  and  00  P(i  10). 
Its  fracture  is  conchoidal.  The  mineral  is  white,  gray  or  colorless  and 
transparent,  and  is  often  tarnished  with  a  gray  coating.  It  has  an 
adamantine  or  residuous  luster,  is  brittle  and  has  a  colorless  streak. 
Its  hardness  is  2/5-3  and  sp.  gr.  6.3-6.4.  Impure  varieties  may  be 
tinged  with  yellow,  green  or  blue  shades  and  in  some  cases  may  be 
opaque.     Its  refractive  indices  for  yellow  light  are  :  a  =  1 .877 1 ,  7  =  1 .893  7. 

Before  the  blowpipe  anglesite  decrepitates.  It  fuses  in  the  flame  of 
a  candle.  On  charcoal  it  effervesces  when  heated  with  the  reducing 
flame  and  yields  a  button  of  metallic  lead.     In  the  oxidizing  flame  it 


SULPHATES 


243 


gives  the  lead  sublimate.     The  mineral  dissolves  in  HNO3  with  dif- 
ficulty. 

The  mineral  is  characterized  by  its  high  specific  gravity  and  the 


m 


m 


Fig.  136.- 
Fig.    137.- 


Fig.  136.  Fig.  137. 

-Anglesite  Crystal  with    <*>  P,  no  (m);     00  P  qo  ,  xoo  (a);    oP,  001   (c); 

}P,  112  (r)  and  PX  122  (y). 

-Anglesite   Crystal    with   m,  a  and   y  as  in   Fig.    136.    Also    00 Poo, 
cio  l&},  P  00 ,  011  (0);   P,  in  (s)  and  JP  00  ,  102  (d). 


reaction  for  lead.     It  is  distinguished  from  cerussite  by  the  reaction  for 
sulphur  and  the  lack  of  effervescence  with  HCl. 

Syntheses. — Crystals  of  anglesite  have  been  made  by  methods  anal- 
ogous to  those  used  in  the  preparation  of  barite  crystals. 

Occurrence. — The  mineral  occurs  as  an  alteration  product  of  galena, 
mainly  in  the  upper  portions  of  veins  of 
lead  ores.     Under  the  influence  of  solu- 
tions of  carbonates  it  changes  to  cerus- 
site. 

Localities. — It  is  found  in  Derby- 
shire and  Cumberland,  in  England; 
near  Siegen,  in  Prussia;  in  Australia  and 
in  the  Sierra  Mojada,  in  Mexico.  In  the 
United  States  crystals  occur  at  Phoenix- 

ville,  Penn.,  in  the  lead  districts  of   the  Mississippi  Valley,  and  at 
various  points  in  the  Rocky  Mountains. 

Uses. — It  is  mined  with  other  lead  compounds  as  an  ore  of  this  metal. 

BASIC  SULPHATES 

Although  several  basic  sulphates  are  known  as  minerals,  only  two 
are  of  importance.  One,  brochantite,  is  a  copper  compound  found,  with 
other  copper  minerals,  in  the  oxidized  portions  of  ore  veins,  and  the 
other,  alunite,  is  a  double  salt  of  aluminium  and  potassium.     This  min- 


Fig.  138. — Anglesite  Crystal  with 
m,  y,  c  and  d  as  in  Figs.  136  and 
137.    Also  JP  00  ,  104  (/)  and  P4, 

144  (x). 


244  DESCRIPTIVE  MINERALOGY 

eral  is  one  of  a  series  of  compounds  forming  an  isomorphous  group,  with 
the  general  formula  (R,/,(OH)2)6R/2(S04)4  or  (R'"(OH)2)6R"(S04)4, 
in  which  R'"  =  A1  or  Fe,  R'2  =  K2,  Na2  or  H2  and  R"=Pb. 

Alunite  ((A1(OH)2)gK2(S04)4) 

Alunite,  or  alumstone,  is  a  comparatively  rare  mineral,  but,  because 
of  its  possible  utilization  as  a  source  of  potash,  it  is  of  considerable  in- 
terest.   It  has  long  been  used  abroad  as  a  source  of  potash  alum. 

The  mineral,  when  pure,  contains  38.6  per  cent  SO3,  37.0  per  cent 
A12C>3,  1 1. 4  per  cent  K20  and  13.0  per  cent  H20,  which  corresponds  to 
the  formula  given  above,  or  if  written  in  the  form  of  a  double  salt 
3(Al(OH)2)2S04-K2S04.  The  chemical  composition  of  a  crystalline 
specimen  from  Marysville,  Utah,  is  as  follows: 

SO3    A1203  Fe203  P2O5  K20   Na20  H20+  H20-  Si02        Total 
38.34    37-x8       tr.     .58     10.46     .33      12.90      .09      .22         100.10 

Alunite  occurs  in  hexagonal  crystals  (di trigonal  scalenohedral  class), 
with  an  axial  ratio  of  1  :  1.252.  The  natural  crystals  are  nearly  always 
simple  rhombohedrons,  R(ioTi),  or  R  modified  by  other  rhombohedrons 
and  the  basal  plane.  Because  the  angle  between  the  rhombohedral 
faces  is  about  900  (900  50'),  the  habit  of  the  crystals  is  cubical.  The  . 
mineral  also  occurs  massive,  with  fibrous,  granular  or  porcelain-like 
structure. 

Alunite  is  white,  pink,  gray  or  red,  and  has  a  white  streak.  It  is 
transparent  or  translucent  and  has  a  vitreous  or  nearly  pearly  luster. 
Its  cleavage  is  distinct  parallel  to  oP(oooi),  and  it  has  an  uneven,  con- 
choidal  or  earthy  fracture.  Its  hardness  is  3.5-4  and  its  density = 
2.6-2.75.  Its  indices  of  refraction  for  yellow  light  are:  6=1.592, 
(0=1.572. 

Before  the  blowpipe  the  mineral  decrepitates,  but  is  infusible.  In 
the  closed  tube  it  yields  water  and  at  a  high  temperature  sulphurous  and 
sulphuric  oxides.  Heated  on  charcoal  with  Co(N03)2  it  gives  the  blue 
color  characteristic  of  AI22O3.  It  also  gives  the  sulphur  reaction.  It  is 
insoluble  in  water  but  is  soluble  in  H2SO4.  When  ignited  it  gives  off 
all  its  water  and  three-quarters  of  its  SO4,  the  other  quarter  remaining 
in  K2SO4.  When  the  ignited  residue  is  treated  with  water,  the  potas- 
sium sulphate  dissolves  and  insoluble  AI2O3  is  left.  It  is  upon  this 
latter  reaction  that  the  economic  utilization  of  the  mineral  depends. 

The  mineral  is  characterized  by  its  color  and  hardness  together 
with  the  reactions  for  A1,H20  and  sulphuric  acid. 


SULPHATES  245 

Synthesis. — Crystals  have  been  made  by  heating  an  excess  of  alu- 
minium sulphate  with  alum  and  water  at  2300. 

Occurrence  and  Origin. — The  mineral  occurs  in  seams  or  veins  in 
acid  lavas.  It  is  thought  to  have  been  formed  in  some  instances  by 
the  action  of  sulphurous  vapors  upon  the  rock  forming  the  vein  walls, 
in  other  instances  by  direct  precipitation  from  ascending  magmatic 
waters,  and  in  others  by  the  action  of  descending  H2SO4. 

Localities. — The  principal  known  occurrences  of  alunite  are  at 
Tolfa,  Italy;  at  Bulla  Delah,  New  South  Wales;  on  Milo,  Grecian 
Archipelago,  and  at  Mt.  Dore,  France. 

In  the  United  States  it  is  found  with  quartz  and  kaolin  in  the 
Rosita  Hills,  and  the  Rico  Mts.,  Colo.;  in  the  ore  veins  at  Silverton 
and  Cripple  Creek,  Colo.;  as  a  soft  white  kaolin-like  material  in  the 
ore  veins  at  Goldfield,  Nev.;  as  a  crystalline  constituent  in  the  rocks 
at  Goldfield,  Nev.,  and  Tres  Cerritos,  Cal.,  and  in  the  form  of  a  great 
vein  of  comparatively  pure  material  at  Marysville,  Utah. 

Uses. — In  Australia  alunite  is  calcined  and  then  heated  with  dilute 
sulphuric  acid.  The  mixture  is  then  allowed  to  settle  and  the  clear 
solution  is  drawn  off  and  cooled.  Alum  crystallizes.  The  mother  liquor 
which  contains  aluminium  sulphate,  after  further  treatment  with  the 
calcined  mineral,  is  evaporated  and  the  aluminium  salt  separated  by 
crystallization.  In  the  United  States  it  is  now  (1916)  being  utilized 
as  a  source  of  potash  and  aluminium. 

Brochantite  ((CuOH)2S04  2Cu(OH)2)  occurs  in  groups  of  small 
prismatic  crystals,  in  fibrous  masses  and  in  drusy  crusts.  Its  crystal- 
lization is  orthorhombic  with  a  :  b  :  c=.yy^g  :  1  :  .4871  and  the  angle 
iioAiTo=75°  28'.  Cleavage  is  perfect  parallel  to  00  P  06  (010).  The 
mineral  is  emerald-green  to  blackish  green  and  its  streak  is  light 
green.  It  is  transparent  or  translucent,  and  its  luster  is  vitreous, 
except  on  cleavage  planes  where  it  is  slightly  pearly.  Its  hardness  is 
3.5-4  and  density  3.85.  In  the  closed  tube  it  decomposes,  yielding 
water  and,  at  a  high  temperature,  sulphuric  acid.  It  gives  the  usual 
reactions  for  copper  and  sulphuric  acid.  Brochantite  occurs  in  the 
upper  portions  of  copper  veins  at  many  places,  in  some  of  which  it  was 
formed  by  the  interaction  between  silicates  and  solutions  of  copper 
salts.  In  the  United  States  it  has  been  found  at  the  Monarch  Mine, 
Chaffee  Co.,  Colorado,  at  the  Mammoth  Mine,  Tintic  District,  Utah, 
and  in  the  Clifton-Morenci  Mines,  Arizona, 


246  DESCRIPTIVE  MINERALOGY 

HYDROUS  SULPHATES 

The  hydrous  sulphates  comprise  a  number  of  sulphates  combined 
with  water.  Among  them  are  the  normal  salts  mirabUite  or  glauber 
salt  (Na2S04-  10H2O),  gypsum  (CaSO-r  2H2O),  the  epsomite  and  melan- 
terite  groups  (R"S04-7H20),  chalcanthite  (CUSO45H2O),  and  the 
alum  group  (R'A1(S04)2-  12H2O),  kieserite  (MgS04*H20),  polyhalite 
(K.2MgCa2(S04)4-H20),  and  a  number  of  basic  compounds.  Several 
of  them  are  of  considerable  economic  importance.  They  are  separated 
into  a  normal  group  and  a  basic  group, 

HYDRATED  NORMAL  SULPHATES 

The  hydrated  normal  sulphates  occur  in  crystals,  and  most  of  them 
are  found  also  in  beds  interstratified  with  other  compounds  that  are 
known  to  have  been  precipitated  by  the  evaporation  of  sea  water  or  the 
water  of  salt  and  bitter  lakes.    All  are  soluble  in  water. 

Mirabilite,  or  glauber  salt,  (Na2S04  •  10H2O)  is  a  white,  trans- 
parent to  opaque  substance  occurring  in  monoclinic  crystals  or  as 
efflorescent  crusts.  Its  hardness  is  1.5-2  and  specific  gravity  1.48.  It 
is  soluble  in  water  and  has  a  cooling  taste.  When  exposed  to  the  air  it 
loses  water  and  crumbles  to  a  powder.  Mirabilite  occurs  at  the  hot 
springs  at  Karlsbad,  Bohemia  and  is  obtained  from  the  water  of  many 
of  the  bitter  lakes  in  California  and  Nevada.  Its  crystals  are  deposited 
from  a  pure  solution  of  Na2S04.  If  the  solution  contains  NaCl,  how- 
ever, thenardite  (Na2S04)  deposits. 

Kieserite  (MgS04  •  H2O)  occurs  commonly  in  granular  to  compact, 
massive  beds  interstratified  with  halite  and  other  soluble  salts  at  Stass- 
furt,  Germany,  and  at  other  places  where  ocean  water  has  been  evap- 
orated. It  is  believed  to  have  resulted  from  the  partial  desiccation  of 
epsomite  (MgS04*7H20),  though  it  may  be  deposited  from  a  solution 
of  MgS04  in  the  presence  of  MgCb.  Kieserite  is  white,  gray,  or  yellow- 
ish, and  is  transparent  or  translucent.  It  forms  sharp  bipyramidal 
monoclinic  crystals.  Its  hardness  is  3  and  its  density  2.57.  In  the 
presence  of  water  it  passes  over  into  epsomite  and  dissolves,  yielding  a 
solution  with  a  bitter  taste.  Large  quantities  are  utilized  in  the  fer- 
tilizer industry. 

When  exposed  to  the  air  it  becomes  covered  with  an  opaque  crust. 


SULPHATES 


Gypsum  (CaS04  aH-O) 

Gypsum  is  the  most  important  of  all  the  hydrous  sulphates.  It 
occurs  in  massive  beds  as:sociated  with  limestone,  in  crystals,  in  finely 
granular  aggregates  and  in  fibrous  masses,  under  a  great  variety  of 
conditions. 

Theoretically,  it  consists  of  46.6  per  cent  SO3,  32.5  per  cent  CaO  and 
30.9  per  cent  H2O,  but  usually  it  contains  also  notable  quantities  of  other 
components,  especially  Fe203,  AI2O3  and  Si02-  Clay  is  a  common  im- 
purity in  the  massive  varieties. 

The  analyses  of  two  commercial  gypsums  follow: 


CaSO., 

Dillon,  Kans 78 .  40 

Alabaster,  Mich 78.51 


H20  Si02  AI2O3  CaC03  MgC03  Total 
10.06  .35  .12  .56  .57  99.96 
2096     .05      .08        ...         .11      9971 


The  crystals  are  monoclinic  {prismatic  class),  with  a  :  b  :  £=.6895  : 
1  :  .4132  and  (J=8i°  02'.  They  are  usually  developed  with  a  tabular 
habit  due  to  the  predominance  of  °oPm>(oio).    The  prism  «>P(iio), 


Fie.  139. 
Fio.  139. — Gypsum  Crystals  with  ' 


w 

Fio.  140. 
a  (ft);    -P,  in  (0  and 


o(«i);    =»P 
JP 5>,  T03  W. 
FlC.    i40.r~<iypmni  Twinned  about    »P»    (1°°).     Swallow-tail  Twin.     Form  tn, 
/  and  b  as  in  Fig.  J3y. 


and  pyramid  +P(irT)  are  also  nearly  always  present  {Fig.  139).  Often 
the  +P  faces  are  curved,  producing  a  lens-shaped  body.  Twinning  is 
very  common,  giving  rise  to  two  types  of  twinned  crystals.  In  the  most 
common  of  these  00  P  00  {too)  is  the  twinning  plane  and  the  resulting 
twin  has  the  form  of  Fig.  140.  In  the  second  type  — Poo  (101)  is  the 
twinning  plane  {Fig.  141).  Forms  of  this  type  are  frequently  bounded 
by  +PC11T),  -P(iii),  JP*(7oj),  and   00  P do  (100).     When  the  side 


248 


DESCRIPTIVE  MINERALOGY 


faces  are  curved  the  well  known  arrowhead  twins  result  (Fig.  141). 

The  angle  noAiTo=68°  30'. 

The  mineral  possesses  a  good  cleavage   parallel  to    00  P  00  (010) 

yielding  thin  inelastic  foliae,  another  parallel  to  +P(Tn)  and  a  less 

perfect  one  parallel  to  00  P  *  (100). 
It  is  white,  colorless  and  transpar- 
ent when  pure;  gray,  red,  yellow, 
blue  or  black  when  impure.  Its 
hardness  is  1.5-2  and  sp.  gr.=  2.32. 
The  luster  of  crystals  is  pearly  on 
00  P  ob  (010)  and  on  other  surfaces 
vitreous.  Massive  varieties  are  often 
dull.  The  refractive  indices  for  yel- 
low light  are:  a=  1.5205,  0=  1.5226, 

Fig.    141. — Gypsum    Twinned     about    '"    '^  ' 

-P  «(ioi).  Forms:  °°  P  ao ,  100  In  tne  closed  tube  the  mineral 
(a);  -P,  in  (/);  P,  11T  («)  and  gives  off  water  and  falls  into  a  white 
JPq6,To3(*).    Arrow  head  Twin,      powder  (see  p.   238).     It  colors  the 

flame  yellowish  red  and  yields  the  sul- 
phur test  on  a  silver  coin.  It  is  soluble  in  about  450  pts.  of  water  and 
is  readily  soluble  in  HC1.  When  heated  to  between  2220  F.  and  4000  F. 
it  loses  water  and  disintegrates  into  powder,  which,  when  ground, 
becomes  "  plaster  of  Paris."  This,  when  moistened  with  water,  again 
combines  with  it  and  forms  gypsum.  The  crystallization  of  the  mass 
into  an  aggregate  of  interlocking  crystals  constitutes  the  "  set." 

Gypsum  is  distinguished  from  other  easily  cleavable,  colorless  min- 
erals by  its  softness  and  the  reactions  for  S  and  H2O. 

The  varieties  of  gypsum  generally  recognized  are: 

Selenite,  the  transparent  crystallized  variety; 

Satins  par,  a  finely  fibrous  variety; 

Alabaster,  a  fine-grained  granular  variety,  and 

Rock-gypsum,  a  massive,  structureless,  often  impure  and  colored 
variety. 

Gypsite  is  gypsum  mixed  with  earth. 

Syntheses. — Crystals  of  gypsum  separate  from  aqueous  solutions  of 
CaS04  at  ordinary  temperatures,  and  also  from  solutions  saturated 
with  NaCl  and  MgCb.     Some  of  these  are  twinned. 

Occurrence  and  Origin. — Gypsum  forms  immense  beds  interstrati- 
fied  with  limestone,  clay  and  salt  deposits  where  it  has  been  precipitated 
by  the  evaporation  of  salt  lakes.  Its  crystals  occur  around  volcanic 
vents,  where  they  are  produced  by  the  action  of  sulphuric  acid  on  cal- 


SULPHATES  249 

careous  rocks.  They  are  also  found  isolated  in  day  and  sand,  and  in 
limestone,  wherever  this  rock  has  been  acted  upon  by  the  sulphuric  acid 
resulting  from  the  weathering  of  pyrite.  Gypsum  also  occurs  in  veins 
and  is  found  in  New  Mexico  in  the  form  of  hills  of  wind-blown  sand. 

Localities. — Crystals  are  found  in  the  salt  beds  at  Bex,  Switzerland; 
in  the  sulphur  mines  at  Girgenti,  Sicily,  and  at  Montmartre,  France. 
In  the  United  States  they  occur  at  Lockport,  N.  Y.,  in  Trumbull  Co., 
Ohio,  and  in  Wayne  Co.,  Utah,  in  limestone;  and  on  the  St.  Mary's 
River,  Maryland,  in  clay. 

Extensive  beds  occur  in  Iowa,  Michigan,  New  York,  Virginia,  Ten- 
nessee, Oklahoma  and  smaller  deposits  in  many  other  states,  and  wind- 
blown sands  in  Otero  Co.,  New  Mexico. 

Uses. — Crude  gypsum  is  used  in  the  manufacture  of  plaster,  as  a 
retarder  in  Portland  cement,  and  as  a  fertilizer  under  the  name  of  land 
plaster.  The  calcined  mineral  is  used  as  plaster  of  Paris  and  in  the 
manufacture  of  various  wall  finishing  plasters,  and  certain  kinds  of 
cements.  Small  quantities  are  used  in  glass  factories,  and  as  a  white- 
wash, a  deodorizer,  to  weight  phosphatic  fertilizer,  as  an  adulterant  in 
candy  and  other  foods,  and  as  a  medium  for  sculpture. 

Production. — The  quantity  of  gypsum  mined  in  the  United  States 
during  1912  aggregated  2,500,757  tons,  valued  at  $6,563,908  in  the  form 
in  which  it  was  sold.  Of  this  amount,  441,600  tons  of  crude  material, 
valued  at  $623,500  were  sold  ground,  and  1,731,674  tons,  valued  at  $5,- 
940,409,  were  calcined.  The  output  of  New  York  was  valued  at  $1,241,- 
500,  that  of  Iowa  at  $845,600  and  of  Ohio  at  $812,400. 

After  the  United  States  the  next  largest  producer  is  France  with  a 
product  in  1910  of  1,760,900  tons,  valued  at  $2,942,600  and  Canada  with 
525,246  tons,  valued  at  $934,446. 

EPSOMITE  AND  VITRIOL  GROUPS 

These  groups  comprise  minerals  with  the  general  formula  RSO4  •  7H2O, 
in  which  R=Mg,  Zn,  Fe,  Ni,  Co,  Mn  and  Cu.  Isomorphous  mix- 
tures indicate  that  the  compounds  are  diomorphous,  and  that  the 
group  is,  therefore,  an  isodimorphous  group.  The  group  is  separable 
into  two  divisions,  of  which  one,  the  epsomite  group,  crystallizes  in  the 
bisphenoidal  class  of  the  orthorhombic  system  with  axial  ratios  approx- 
imating 1:1:  .565.  The  other  division,  the  vitriol,  or  melanterite, 
group  crystallizes  in  the  prismatic  class  of  the  monoclinic  system  with 
axial  ratios  approximating  1.18  :  1  :  1.53  and  0  approximating  750. 
Only  the  magnesium  compound  among  the  pure  salts  is  known  to  crys- 
tallize in  both  systems.     Crystals  separated  from  a  saturated  solution 


m 


250  DESCRIPTIVE  MINERALOGY 

are  orthorhombic,  while  those  separated  from  a  supersaturated  solution 
are  monoclinic.  Other  salts  occur  in  isomorphous  mixtures  in  both 
systems.  All  members  of  the  group  are  soluble  in  water  and  all  occur  as 
secondary  products  formed  by  decomposition  of  other  minerals. 

Epsomite  (MgS04  7H2O) 

Epsomite,  or  Epsom  salt,  usually  occurs  in  botryoidal  masses  and 
fibrous  crusts  coating  various  rocks  over  which  dilute  magnesium  sul- 
phate solutions  trickle,  and  mingled  with  earth 
in  the  soils  of  caves.  The  solutions  result  from 
the  action  upon  magnesian  rocks  of  sulphuric 
acid  derived  from  oxidizing  sulphides.  Crys- 
tals are  rare. 

The  composition  corresponding  to  MgS04  • 
I     i  7H2O  demands  32.5  SO3,  16.3  MgO  and  51.2 

^7       H2°- 

The  mineral  forms  white  or    colorless  bi- 
Fig.  i42.~Epsomite  Crys-  sphenoidal,    orthorhombic    crystals,    with    an 

tal    with    00P,    no  (m)        .  .  .  ...  .  «,.    . 

p  axial  ratio  alb:  c=.oooi  :  1  :  .5709.     Their 

and  -r,  in  (s).  habit  is  tetragonal.    The  angle  no  A  110=89° 

26'.    The  commonest  forms  occurring  on  syn- 

P  P 

thetic  crystals  are  combinations  of  ooP(no),  and  — r(in)  or  —/(in) 

2  2 

(Fig.   142).     Natural   crystals  contain,   in  addition  00 Poo  (010)  and 

P  06(101). 

The  luster  of  epsomite  is  vitreous,  its  hardness  2.0-2.5  and  specific 
gravity  1.70.  Its  refractive  indices  for  yellow  light  are:  a=  1.4325, 
0=  1.4554  and  7=  1.4608. 

The  mineral  is  soluble  in  water,  yielding  a  solution  with  a  bitter  taste. 
With  a  solution  of  barium  chloride  it  yields  a  white  precipitate  of  BaS04. 

Epsomite  is  distinguished  from  other  colorless,  soluble  minerals  by 
its  taste  and  the  reactions  for  S  and  Mg. 

Synthesis. — Crystals  are  produced  by  evaporation  of  solutions  of 
MgS04  containing  certain  other  salts.  From  those  containing  borax, 
crystals  of  the  type  indicated  above  are  separated.  The  production  of 
right  or  left  crystals  may  be  provoked  by  inoculation  of  the  solution  with 
a  particle  of  a  crystal  of  the  desired  type. 

Localities. — Epsomite  occurs  in  mineral  waters,  as,  for  instance,  at 
Seidlitz,  Bohemia,  on  the  walls  of  mines  and  caves,  among  the  deposits 
of  bitter  lakes,  and  as  crystals  in  the  soil  covering  the  floors  of  caves. 


SULPHATES  251 

Melanterite,  or  copperas  (FeS04  7H2O),  is  usually  in  fibrous, 
stalactitic  or  pulverulent  masses  associated  with  pyrite  or  other  sul- 
phides containing  iron,  from  which  it  was  produced  by  weathering 
processes.  It  is  commonly  some  shade  of  green.  Its  streak  is  colorless. 
Its  crystals,  which  are  monoclinic  (prismatic  class),  are  rare.  The 
mineral  has  a  hardness  of  2  and  a  density  of  1.9.  It  is  soluble  in  water, 
forming  a  solution  which  has  a  sweetish  astringent  taste. 

ALUM  GROUP 

The  alum  group  includes  a  large  number  of  isomorphous  compounds 
with  the  general  formula  R'Al(SC>4)2-i2H20.  The  group  crystallizes 
in  the  isometric  system  (dyakisdodecahedral  class),  but  all  of  its  mem- 
bers are  so  readily  soluble  in  water  that  they  are  rarely  found  in  nature. 
The  commonest  alums  are  kalinite  (KAl(S04)2-i2H20)  and  soda  alum 
(NaAl(S04)2-i2H20). 

DOUBLE  SULPHATES  WITH  CARBONATES  OR  CHLORIDES 

A  number  of  compounds  of  sulphates  with  chlorides  and  carbonates 
are  known,  but  of  these  only  one  is  of  any  great  economic  importance. 
Two  others  afford  interesting  crystals.  The  commercial  compound  is 
kainite,  which  is  a  hydrated  combination  of  MgS04  and  KC1,  with 
the  formula  MgS04  •  KG  •  3H2O.  The  other  two  best  known  members 
of  the  group  are  leadhillite  (PbS04-Pb(PbOH)2(C03)2  and  hanksite 
(2Na2C03  •  oNa2S04  •  KC1). 

Kainite  (MgS04  KC1  3H20) 

Kainite  is  found  only  in  beds  associated  with  halite  and  other  deposits 
from  saline  waters.  It  is  rarely  crystallized.  Crystals  are  monoclinic 
(prismatic  class),  with  a:6:c=i.2i86:i:  .5863  and  0=85°  6'.  They 
possess  a  pyramidal  habit  with  oP(ooi)  and  ±P(ni)(iiT)  predom- 
inating. 

The  mineral  usually  forms  granular  masses  which  are  white,  yellow, 
gray  or  red.  It  is  transparent,  has  a  hardness  of  2  and  sp.  gr.  2.13, 
and  is  easily  soluble  in  water.  Its  refractive  indices  for  sodium  light  are: 
a=  1.4948  and  7  =  1.5203. 

When  heated  in  a  glass  tube  it  yields  water  and  HC1.  It  is  distin- 
guished from  other  soluble  minerals  by  this  reaction,  and  by  the  fact 
that  it  yields  the  test  for  sulphur,  and  colors  the  flame  blue  when  its 
powder  is  mixed  with  CuO  and  heated  before  the  blowpipe. 


252  DESCRIPTIVE  MINERALOGY 

Synthesis. — Crystals  have  been  produced  by  evaporating  a  solution 
of  K2SO4  and  MgSC>4  containing  a  great  excess  of  MgCi2. 

Occurrence. — Kainite  occurs  in  the  salt  beds  of  Stassfurt,  Germany, 
and  of  Kalusz  in  Galicia,  and  in  the  deposits  of  salt  lakes  and  lagoons. 
It  also  occurs  as  crusts  on  some  of  the  lavas  of  Vesuvius. 

Uses. — The  mineral  is  utilized  as  a  source  of  potassium  in  the  manu- 
facture of  potassium  salts  and  fertilizers.  Large  quantities  are  imported 
annually  into  the  United  States.  In  191 2  the  imports  aggregated 
485,132  tons,  valued  at  $2,399,761. 


Hanksite  (2Na2C03  9Na2S04  KC1)  occurs  almost  exclusively  in 

hexagonal  prisms  that  are  prismatic  or  tabular, 
or  in  double  pyramids  suggesting  quartz  crys- 
tals. Their  axial  ratio  is  1  :  1.006.  The  com- 
monest crystals  are  bounded  by  oP(oooi), 
Fig.  143.— Hanksite  Crys-  ooP(ioTo),  P(ioTi)  (Fig.  143)  and  2P(202i), 
tal  with  cop,  1010  (m);  or   ^(4045).      Their    cleavage    is    imperfect 

P,  ion  (0)  and  oP,  0001  hi-        t>/         \       <™.         •         1  •       v-^ 

/  v  parallel  to  oP(oooi).     The  mineral  is  white  or 

yellow.      Its    hardness    =2    and    its   specific 

gravity  =2.56.     It  is  soluble  in  water.     Its  refractive  indices  are: 

o)=  1.4807  and  €=1.4614.    It  occurs  at  Borax  Lake  and  Death  valley, 

California,  in  the  deposits  of  salt  lakes. 


Leadhillite  (PbS04Pb(PbOH)2(C03)2)  occurs  principally  as 
crystals  in  the  oxidized  zones  of  lead  and  silver  veins.  The  crys- 
tals are  monoclinic  (prismatic  class),  and  have  an  hexagonal  habit. 
Their  axial  ratio  is  1.7515  :  1  :  2.2261.  /3=89°32'.  The  principal 
forms  observed  on  them  are  oP(ooi),  ooP(no),  00 Poo  (100),  P(m) 
and  §P6o  (102).  In  the  most  common  twins  ooP(no)  is  the  twin- 
ning plane.  The  mineral  is  white  or  yellow,  green  or  gray,  and  it  is 
transparent  or  translucent.  Its  streak  is  colorless.  It  is  sectile,  has  a 
hardness  of  2.5  and  a  specific  gravity  of  6.35.  Before  the  blowpipe  it 
intumesces,  turns  yellow,  and  fuses  easily  (1.5).  Upon  cooling  it  again 
becomes  white.  It  effervesces  in  HNO3  and  leaves  a  white  precipitate 
of  PbS04.  It  reacts  for  sulphur  and  water.  It  is  found  at  Leadhills, 
Scotland,  and  Mattock,  England,  associated  with  other  ores  of  lead; 
at  a  lead  mine  near  Iglesias,  Sardinia,  and  at  several  silver-lead  mines 
in  Arizona. 


CHAPTER   XIII 


THE  CHROMATES,  TUNGSTATES  AND  MOLYBDATES 


THE  CHROMATES 

The  only  chromate  of  importance,  among  minerals,  is  the  lead  salt  of 
normal  chromic  acid,  H2C1O4.  There  are  several  other  chromates 
known,  but  they  are  basic  salts  and  are  rare.  All  are  lead  compounds. 
The  normal  salt,  PbCrCU,  is  known  as  crocoite.  Chromic  acid  is  un- 
known, as  it  spontaneously  breaks  down  into  C1O3  and  water  when  set 
free  from  its  salts.  Its  best  known  compound  is  potassium  chromate, 
K^CrCU. 

Crocoite  (PbCr04) 

Crocoite  is  well  characterized  by  its  hyacinth-red  color.  It  is  a  lead 
chromate  with  PbO=68.g  per  cent  and  003  =  31.1  per  cent. 

Its  crystallization  is  monoclinic 
(prismatic  class)  with  a  :  b  :  c 
=  .9603  :  1  :  .9159  and  £=77°  33'. 
Its  crystals,  which  are  usually  im- 
planted on  the  walls  of  cracks  in 
rocks,  are  prismatic  or  columnar 
parallel  to  ooP(no).  Their  pre- 
dominant forms  are  oop(no), 
— P(iii),  and  various  domes  (Fig. 
144).  Their  cleavage  is  distinct 
parallel  to  ooP(no).  The  angle 
iioAiio=86°  19'.  The  mineral 
also  occurs  in  granular  masses. 

Crocoite  is  bright  hyacinth-red, 
and  is  translucent.  Its  streak  is 
orange-yellow.  The  mineral  is  sec- 
tile.  Its  fracture  is  conchoidal,  its 
hardness  2.5-3  and  density  about  6. 


Fig.  144. — Crocoite  Crystals  with  00  Pf 
no  (m);  00  P2,  120  (/);  -P,  in  (/); 
3P*.  301  (*);  P«,  101  (*);  oP, 
001  (c);  Pob,  on  («);  2Pobfo2i  (y) 
and  iPob  ,  012  (u>). 

Its  intermediate  refractive  index 


is  about  2.42. 

In  the  closed  tube  it  decrepitates,  and  blackens;  but  it  reassumes  its 
red  color  when  heated.    On  charcoal  it  deflagrates  and  fuses  easily, 

253 


254  DESCRIPTIVE  MINERALOGY 

yielding  metallic  lead  and  a  lead  coating.  With  microcosmic  salt  it 
gives  the  green  bead  of  chromium. 

The  mineral  is  easily  recognized  by  its  color  and  the  test  for  chro- 
mium. 

Synthesis. — Crystals,  like  those  of  crocoite,  have  been  obtained  by 
heating  on  the  water  bath  a  solution  of  lead  nitrate  in  nitric  acid  and 
adding  a  dilute  solution  of  potassium  bichromate.  ' 

Occurrence. — Crocoite  occurs  under  conditions  which  suggest  that  it 
is  a  product  of  pneumatolysis. 

Localities. — It  is  found  in  the  Urals;  at  Rezbanya  and  Moldawa,  in 
Hungary;  in  Tasmania,  and  in  the  Vulture  Mining  district,  Maricopa 
Co.,  Arizona. 

THE  TUNGSTATES  AND  MOLYBDATES 

The  tungstates  are  salts  of  tungstic  acid,  H2WO4.  They  are  the 
principal  sources  of  the  metal  tungsten  which  is  beginning  to  have  im- 
portant uses.  The  molybdates  are  salts  of  moiybdic  acid,  H2M0O4. 
The  two  most  prominent  tungstates  are  scheelite,  CaWCk,  and  wolf- 
ramite (Fe-Mn)W04,  and  the  most  prominent  molybdate  is  wulfenite, 

PbMo04. 

All  tungsten  compounds  give  a  blue  bead  with  salt  of  phosphorus  in 
the  reducing  flame.  When  fused  with  Na2C(>3,  dissolved  in  water 
and  hydrochloric  acid,  and  treated  with  metallic  zinc  (see  pp.  482,  and 
492  for  details  of  test),  they  also  yield  a  blue  solution  which  rapidly 
changes  to  brown. 

The  molybdates  give  with  the  salt  of  phosphorus  bead  in  the  oxidiz- 
ing flame  a  yellow-green  color  while  hot,  changing  to  colorless  when  cold. 
In  the  reducing  flame  the  color  is  clear  green. 

SCHEELITE   GROUP 

The  scheelite  group  comprises  a  series  of  tungstates  and  molybdates 
of  Ca,  Cu  and  Pb.  The  minerals  are  tetragonal  and  hemihedral  and 
are  all  well  crystallized.  The  more  important  members  of  the  group 
are  scheelite  and  wulfenite.  Cuprotungstite  is  a  copper  tungstate  (CuWCU) 
and  stolzite  a  lead  tungstate  (PbW04). 

Scheelite  (CaW04) 

The  formula  of  scheelite  demands  80.6  per  cent  WO3,  and  19.4  per 
cent  CaO,  but  the  mineral  usually  contains  a  little  molybdenum  in 
place  of  some  of  the  tungsten.     It  nearly  always  contains  also  a  little  Fe. 


CHROMATES,   TUNGSTATES  AND  MOLYBDATES  255 

Scheelite  crystallizes  in  the  tetragonal  bipyramidal  class.  Its  crys- 
tals are  usually  pyramidal,  though  often  tabular  in  habit.  Their  axia! 
ratio  is  i  :  1.5268.  On  the  pyramidal  types  the  predominant  planes 
are  pyramids  of  the  first,  second  (Fig.  145),  and  third  orders  and  on  the 
tabular  types,  in  addition,  the  basal  plane.    One  of  the  most  familiar 

combinationsisPtxxO.Poo  (loI).  ^(3.3)  and  [^](130  (Fig.  145). 

Other  forms  frequently  found  on  its  crystals  are  £P  oo  (102)  and  £P  °° 
(105).  The  angle  iioaTii  =  79°  55 J'..  Twinning  is  common,  both 
contact  and  penetration  twins  having  °o  P  00  (100)  as  the  twinning 
plane.     The  mineral  also  occurs  in  reniform  and  granular  masses. 

Scheelite  is  white,  yellow,  brown,  greenish  or  reddish,  with  a  white 


Fig.  145.  Fig.  146. 

Fig.  145. — Scheelite  Crystal  with  P,  n  1  \p);  Poo  ,  101  (e)  and  oP,  001  (c). 

Fig.  146. — Scheelite  Crystal  with  P  and  e  as  in  Fig.  145.    Also  I       1 ,  313  (h)  and 

f3p3l  ,  n  L  2  J 

L"TJ»  r3x  W- 

• 

streak  and  vitreous  luster.  It  has  a  distinct  cleavage  parallel  to  P(ooi), 
and  an  uneven  fracture.  It  is  brittle,  has  a  hardness  of  4.5-5  an(i  a 
density  of  about  6,  and  is  transparent  or  translucent.  It  is  soluble  in 
HC1  and  HNO3  with  the  production  of  a  yellow  powder,  tungsten  tri- 
oxide,  which  is  soluble  in  ammonia.  Its  refractive  indices  are:  e=  1.9345, 
«=  1.9185  for  red  light. 

Before  the  blowpipe  the  mineral  fuses  to  a  semitransparent 
glass.  With  borax  it  forms  a  transparent  glass  which  becomes  opaque 
on  cooling.  With  salt  of  phosphorus  it  yields  the  characteristic  beads 
for  tungsten,  but  specimens  containing  iron  must  be  heated  with  tin  on 
charcoal  before  the  blue  color  can  be  developed. 

Scheelite  is  distinguished  from  limestone,  which  its  massive  forms 
closely  resemble,  by  its  higher  specific  gravity  and  the  absence  of  effer- 


266  DESCRIPTIVE  MINERALOGY 

vescence  with  HC1.  From  quartz  it  is  distinguished  by  its  softness  and 
from  barite  by  greater  hardness  and  higher  specific  gravity. 

Syntheses. — Crystals  of  scheelite  have  been  made  by  adding  a  solu- 
tion of  sodium  tungstate  to  a  hot  acid  solution  of  CaCk,  and  by  fusing 
the  two  compounds.  They  have  also  been  produced  by  fusing  wolfram- 
ite with  CaCl2. 

Occurrence  and  Origin. — Scheelite  is  found  in  gold-quartz  veins 
and  in  veins  cutting  acid  igneous  rocks,  where  it  is  associated  with 
cassiterite,  topaz,  fluorite,  molybdenite,  wolframite  and  many  other 
metallic  compounds,  and  as  a  contact  metamorphic  product  in  altered 
limestone  intruded  by  granite.  It  is  probably  in  all  cases  a  deposit 
from  hot  solutions. 

Localities. — It  occurs  at  Zinnwald,  Bohemia;  Altenberg,  Saxony; 
Carrock  Fells,  Cumberland,  England;  Pitkaranta,  Finland;  in  New 
Zealand;  and  in  the  United  States  at  Monroe  and  Trumbull,  Conn.;  in 
the  Atolia  District,  Kern  Co.,  California;  the  Mammoth  Mining  Dis- 
trict, Nevada;  in  Lake  County,  Colorado;  near  Gage,  New  Mexico, 
where  it  occurs  with  pyrite  and  galena  in  a  vein  cutting  limestone, 
and  in  the  placer  gravels  at  Nome,  Alaska. 

Uses  of  Tungsten. — Tungsten  is  used  principally  in  the  manufacture 
of  tool  steel,  electric  furnaces  and  targets  for  Rontgen  rays.  It  is 
employed  also  as  filaments  in  electric-light  bulbs,  in  the  manufacture 
of  sodium  tungstate  which  is  used  for  fireproofing  cloth,  as  a  mordant 
in  dyeing,  and  for  a  number  of  other  minor  purposes. 

Production. — Scheelite  has  been  mined  in  small  quantity  in  Idaho, 
Alaska,  California,  Nevada,  Arizona,  and  New  Mexico,  as  a  source  of 
tungsten,  but  most  of  this  element  has  heretofore  been  produced  from 
other  compounds,  mainly  wolframite.  In  191 3  a  few  hundred  tons  of 
scheelite  concentrates  were  produced  in  the  Atolia  district,  California, 
and  the  Old  Hat  district,  near  Tucson,  Ariz.  At  present  (1916)  it  is 
being  produced  in  large  quantity  near  Bishop,  Inyo  Co.,  Cal. 

Stolzite  (PbW04)  is  completely  isomorphous  with  wulfenite.  Its 
crystals,  which  are  pyramidal  or  short  columnar,  are  mainly  combina- 
tions of  ooP(no),  P(iii),  2P(22i)  and  oP(ooi).  Their  axial  ratio  is 
1  :  1.5606. 

The  mineral  is  gray,  brown,  green  or  red.  It  is  translucent  and 
has  a  white  streak.  Its  hardness  is  2.75-3  an(^  *ts  SP-  6r-  7-87-8.23. 
Its  refractive  indices  for  yellow  light  are:  w  =  2.2685,  €  =  2.182. 

Before  the  blowpipe  it  decrepitates  and  melts  to  a  lustrous  crystal- 
line globule.    The  bead  with  microcosmic  salt  in  the  reducing  flame 


CHROMATES,  TUNGSTATES  AND  MOLYBDATES   257 

is  blue  when  cold;  in  the  oxidizing  flame  it  is  colorless.  The  mineral 
is  decomposed  by  HNO3  leaving  a  yellow  residue  of  WO3.  Crystals 
have  been  made  by  fusing  sodium  tungstate  and  lead  chloride. 

Its  principal  localities  are  the  tin-bearing  veins  at  Zinnwald,  Bo- 
hemia; the  copper  veins  in  Coquimbo,  Chile;  and  Southampton,  Mass., 
where  it  is  associated  with  other  lead  compounds. 

Wulfenite  (PbMoCU) 

Wulfenite  is  the  only  raolybdate  of  importance  that  occurs  as  a 
mineral.  Its  formula  demands  39.3  M0O3  and  60.7  PbO.  Calcium 
sometimes  replaces  a  part  of  the  Pb  and  tungsten  a  part  of  the  Ma 

Wulfenite  is  hemihedral  and  hemimorphic  (tetragonal  pyramidal 
class).  Its  crystals  are  more  frequently  tabular  than  those  of  scheelite, 
and  they  are  usually  very  thin. 

The  mineral,  however,  occurs  also  in  pyramidal  and  prismatic  crys- 
tals which,  in  some  cases,  exhibit  distinct  hemimorphism.    Their  axial 


<^i 


Fig.  147.  Fig.  148. 

Fig.  147. — Wulfenite  Crystal  with  00  p  00 ,  100  (a)  and  ^P  00  , 1.0.12  (0). 
Fig.   148. — Wulfenite   Crystal  with  oP,  001  (c);    JP°ot   102  (w);   P«,   ioi  (e); 

P,  hi  (n)  and  JP,  113  (5). 

ratio  is  a : c=i :  1.5777.    The  most  common  forms  found  on  its  crys- 
tals are:  oP(ooi),  P(in),  |-^— ^(320),  £P(ii3)  and  P 00(101)  (Fig. 

147  and  148).    The  angle  1 1 1 A  Ti  1  =  8o°  22'. 

The  cleavage,  parallel  to  P,  is  very  smooth,  and  the  fracture  is  con- 
choidal.  The  mineral  is  brittle.  Its  hardness  is  about  3  and  specific 
gravity  about  6.8.  Its  luster  is  resinous  or  adamantine,  and  its  color 
orange-yellow,  olive-green,  gray,  brown,  bright  red  or  colorless.  Its 
streak  is  white  and  it  is  transparent.     For  red  light,  co=  2.402,  €=  2.304. 

Before  the  blowpipe  wulfenite  decrepitates  and  fuses  readily.  With 
salt  of  phosphorus  it  gives  the  molybdenum  beads.  With  soda  on 
charcoal  it  yields  a  lead  globule.  When  the  powdered  mineral  is  evap- 
orated with  HC1  molybdic  oxide  is  formed.  On  moistening  this  with 
water  and  adding  metallic  zinc  an  intense  blue  color  is  produced. 

Wulfenite  is  distinguished  from  lanadinite  (p.  271),  by  crystalliza- 
tion, by  the  test  for  chlorine  (vanadinite)  and  the  test  for  tungsten. 


258     .  DESCRIPTIVE  MINERALOGY 

Synthesis. — Wulfenite  crystals  have  been  produced  by  melting 
together  sodium  molybdate  and  lead  chloride. 

Occurrence  and  Localities. — The  mineral  occurs  in  the  oxidized  zone 
of  veins  of  lead  ores  at  some  of  the  principal  lead  occurrences  in  Europe, 
end  in  the  United  States  near  Phoenixville,  Pennsylvania;  in  the  Organ 
Mountains,  New  Mexico;  at  the  mines  in  Yuma  County,  Arizona;  at 
the  Mammoth  Mine,  in  Pinal  County  in  the  same  State,  and  at  many 
other  of  the  lead  mines  in  the  Rocky  Mountain  states. 

Uses. — Wulfenite  is  an  important  source  of  molybdenum,  but, 
because  of  the  few  uses  to  which  this  metal  is  put,  the  amount  of  wulfen- 
ite mined  annually  is  very  small. 


~     *   • 


WOLFRAMITE  GROUP 

Wolframite  ((Fe  Mn)W04) 

Wolframite  is  the  name  given  the  isomorphous  mixture  of  the  man- 
ganese and  iron  tungstates  that  occur  nearly  pure  in  some  varieties 
of  the  minerals  hiibnerite  and  ferberite. 

The  mixture  of  the  iron  and  manganese  molecules  is  more  common 
than  either  alone,  consequently  wolframite  is  the  commonest  member  of 
the  group.  The  properties  of  all  three  minerals,  however,  are  so  nearly 
alike  that  they  must  be  distinguished  by  chemical  analysis. 

The  name  wolframite  is  usually  applied  to  mixtures  of  the  tungstates 
in  which  the  proportion  of  Fe  to  Mn  varies  between  4  :  1  and  2  :  3,  or 
between  9.5  per  cent  and  18.9  per  cent  of  FeO  and  14  per  cent  and 
4.7  per  cent  of  Mn02. 

It  has  recently  been  suggested  that  the  name  ferberite  be  limited 
to  mixtures  containing  not  more  than  20  per  cent  of  the  hiibnerite  mole- 
cule and  the  name  hiibnerite  to  those  containing  not  more  than  20  per 
cent  of  the  ferberite  molecule.  This  would  leave  the  name  wolframite 
for  mixtures  containing  more  than   20  per  cent  of  both  FeW04  and 

MnW04. 

Analyses  of  specimens  of  hiibnerite  (I),  wolframite  (II  and  III) 
and  ferberite  (IV)  follow: 

WO3  FeO  MnO  CaO  Other  Total 

I.  Ellsworth,  Nye  Co.,  Nev. .  .      74.88  .56  23.87  .14  .16  99.61 

II.  Sierra  Cordoba,  Argentine . .      74.86  13.45  11.02  ...  1.22  100.55 

III.  Cabarrus  Co.,  N.  C 75-79  19.80  5.35  .32  tr  101.26 

IV.  Kimbosan,  Japan 75-47  24.33  •••■  tr  tr  99.80 

All  members  of  the  group  crystallize  in  the  monoclinic  system 
(prismatic  class)  with  axial  ratios  as  follows: 


CHROMATES,  TUNGSTATES  AND  MOLYBDATES  259 


Ferberite       a : 

:b: 

^=.8229  : 

1  : 

.8463. 

0- 

=  89° 

38' 

Wolframite 

=  .8300  : 

1  : 

.8678. 

0= 

=  89° 

38' 

Hiibnerite 

=  •8315^ 

1  : 

.8651. 

0= 

=  89° 

38' 

Fig.  149. — Wolframite  Crys- 
tal with  00  P,  no  (m); 
00  P2,  210  (/);  oop56, 
100  (a);  -JP06,  102  (0; 
P«,on  (/);  —  2P2, 121 
(a);  +JP«  ,  102  (>')  and 
— P,  in  (w). 


The  crystals  are  prismatic  or  cubic  in  habit  and  are  bounded  by 
ooP(no),  00  Poo  (100),  and  two  or  more  of  the  following:  oP(ooi), 

00  P  06  (OIO),  00  P5(2I0),  P  ob  (oil),  £P  6b  (T02),  -£P  56  (102),  -P(lll), 

—  2P2(i2i)  and  +2P00  (102)  (Fig.  149).  The 
angle  110A110  for  ferberite=78°  51',  for  wol- 
framite 790  23',  and  for  hiibnerite  790  29'. 
Twins  are  fairly  common,  with  00  P  60  (100) 
the  twinning  plane.  Cleavage  is  perfect 
parallel  to  00  P  ob  (010).  The  minerals  also 
occur  in  lamellar  and  granular  masses. 

Hiibnerite  is*  brownish  red  to  black  and 
translucent,  wolframite  is  black  and  trans- 
lucent only  on  thin  edges,  and  ferberite  is 
black  and  opaque.  The  streak  is  yellow  to 
yellowish  brown  in  hiibnerite  and  brown  or 
brownish  black  in  ferberite,  with  the  streak 
of  wolframite  between. 

Wolframite  is  brittle,  has  a  hardness  of 
5-5.5,  a  specific  gravity  of  7.2-7.5,  and  a  submetallic  luster.  Before 
the  blowpipe  it  fuses  to  a  globule  which  is  magnetic.  Fused  with 
soda  and  niter  on  platinum  it  gives  the  bluish  green  manganate.  The 
salt  of  phosphorus  bead  is  reddish  yellow  when  hot  and  a  paler  tint 
when  cold.  In  the  reducing  flame  the  bead  becomes  dark  red.  If 
the  mineral  is  treated  first  on  charcoal  with  tin  its  bead  assumes  a 
green  color  on  cooling.  The  mineral  dissolves  in  aqua  regia  with 
the  production  of  the  yellow  tungsten  trioxide.  When  treated  with 
concentrated  H2SO4  and  zinc  it  yields  the  blue  tungsten  reaction. 

Crystals  of  wolframite  are  easily  distinguished  from  crystallized 
columbite  (p.  293),  samarskite  (p.  295),  and  uraninite  (p.  297),  by  dif- 
ferences in  crystallization.  Massive  wolframite  is  distinguished  from 
massive  forms  of  the  other  three  minerals  by  its  more  perfect  cleavage 
and  by  the  reactions  with  the  beads.  Uraninite,  moreover,  contains 
lead.  Wolframite  is  distinguished  from  black  tourmaline  (p.  434)  by 
the  differences  in  specific  gravity. 

Occurrence  and  Origin. — Wolframite  usually  occurs  in  veins  with  tin 
ores,  and  in  quartz  veins  with  various  sulphides,  and  in  pegmatite. 
Its  origin  is  probably  pneumatolytic. 


260  DESCRIPTIVE  MINERALOGY 

Localities. — Wolframite  is  found  in  all  tin-producing  districts,  espe- 
cially at  Zinnwald,  Schneeberg  and  Freiberg,  in  Germany;  at  Ner- 
chinsk, in  Siberia;  in  Cornwall,  England;  at  Oruro,  in  Bolivia,  and  at 
various  points  in  New  South  Wales,  Australia. 

In  the  United  States  it  occurs  at  Monroe,  Conn.;  near  Mine  La 
Motte,  Missouri;  near  Lead,  South  Dakota,  where  it  impregnates  a 
sandy  dolomite,  and  at  Htil  City  in  the  same  State  in  quartz  veins, 
sometimes  containing  cassiterite;  in  Boulder  Co.,  Colorado,  in  veins 
in  granite  (ferberite);  near  Butte,  Montana,  in  quartz  veins  carry- 
ing silver  ores  (hubnerite) ;  and  the  quartz-cassiterite  veins  near  Nome 
and  on  Bonanza  Creek,  in  Alaska;  and  in  quartz  veins  at  various 
points  in  Washington,  Idaho,  California,  Nevada,  New  Mexico  and 
Arizona.  At  some  of  these  localities  the  mineral  is  more  properly 
hubnerite. 

One  or  another  of  the  three  has  been  mined  in  Colorado,  Nevada, 
South  Dakota,  Montana,  Washington,  California,  Arizona,  and  New 
Mexico,  but  the  total  production  has  never  been  large.  Some  of  the 
ore  shipped  has  been  obtained  from  placers  along  streams  that  drain 
regions  containing  the  mineral  in  veins,  but  most  of  it  has  been  obtained 
from  vein  rock  which  is  crushed  and  concentrated. 

Uses. — These  three  minerals  constitute  the  principal  source  of  tung- 
sten used  in  the  arts.  The  uses  of  the  metal  are  referred  to  under 
scheelite. 

Production. — The  total  production  of  concentrates  containing  60 
per  cent  WO3  in  the  United  States  during  1913  was  1,525  tons,  valued 
at  $640,500.  Of  this,  953  tons  were  ferberite  from  Boulder  Co., 
Colorado.  A  little  hubnerite  was  produced  in  the  Arivica  region,  in 
southeast  California,  at  Dragoon,  Arizona,  at  Round  Mountain,  Nevada, 
and  on  Paterson  Creek,  Idaho.  In  addition,  there  were  imported 
$86,000  worth  of  tungsten-bearing  ores  and  $143,800  worth  of  tung- 
sten metal  and  ferro-tungsten.  The  world's  production  of  tungsten  ore 
in  1912  was  9,115  tons. 


CHAPTER  XIV 

THE  PHOSPHATES,   ARSENATES   AND   VANADATES 

The  phosphates  are  salts  of  phosphoric  acid,  H3PO4,  the  arsenates 
of  the  corresponding  arsenic  acid,  H3ASO4,  and  the  vanadates  of  the 
corresponding  vanadic  acid,  H3VO4.  The  phosphates  are  by  far  the 
most  important  as  minerals.  They  are  easily  distinguished  by  yielding 
phosphine,  H3P,  upon  igniting  with  metallic  magnesium  and  moistening 
the  resulting  MgaP2  with  H20  or  HC1  (Mg3P2+6HCl  =  3MgCl2+ 
2PH3).  The  gas  is  recognized  by  its  disagreeable  odor.  The  arsenates 
are  detected  by  the  test  for  arsenic. 

The  arsenates,  phosphates  and  vanadates  form  groups  of  isomor- 
phous  compounds,  the  most  important  of  which  is  the  apatite  group. 
Those  occurring  as  minerals  are  divisible  into  several  subgroups,  of 
which  the  following  six  contain  common  minerals,  viz.:  (1)  anhydrous 
(a)  normal  salts,  (b)  basic  salts  and  (c)  acid  salts,  and  (2)  hydrous 
(a)  normal  salts,  (b)  basic  salts  and  (c)  acid  salts. 

A  number  of  the  phosphates  and  arsenates  are  of  value  commercially 
either  because  of  the  phosphorus  they  contain;  because  they  are  sources 
of  valuable  metallic  salts;  because  they  serve  to  indicate  the  presence 
of  other  valuable  compounds;  or  because  they  possess  an  ornamental 
character. 

Nearly  all  the  phosphates  are  transparent  or  translucent  and  all  are 
nonconductors  of  electricity  or  are  very  poor  conductors. 

ANHYDROUS  PHOSPHATES,  ARSENATES  AND 

VANADATES 

NORMAL  PHOSPHATES,  ARSENATES  AND  VANADATES 

The  minerals  belonging  in  this  class  of  compounds  are  not  as  numer- 
ous as  the  basic  salts,  but  some  of  them  are  of  great  value.  The  class 
includes  phosphates  of  yttrium,  the  alkalies,  beryllium,  cerium,  mag- 
nesium, iron  and  manganese  and  a  group  of  isomorphous  phosphates, 
arsenates  and  vanadates — the  apatite  group — in  which  a  haloid  radicle 
replaces  one  of  the  hydrogen  atoms  of  the  acids.  Apatite,  the  prin- 
cipal member  of  the  group,  is  an  important  source  of  phosphoric  acid. 

261 


262  DESCRIPTIVE  MINERALOGY 

Triphylite— (Li(Mn  Fe)P04)— Lithiophilite 

Triphylite  is  the  name  usually  applied  to  the  isomorphous  mixture 
of  LiFeP04  and  LiMnPCU,  in  which  the  manganese  molecule  is  present 
in  small  quantity  only.  The  mixture  containing  a  large  excess  of  the 
manganese  molecule  is  called  lithiophilite. 

The  pure  triphylite  molecule  contains  FeO=45.5  Per  cent>  Li20 
=  9.5  per  cent  and  P20s=45  per  cent.  The  pure  lithiophilite  molecule 
consists  of  45.1  per  cent  MnO,  9.6  per  cent  Li20  and  45.3  per  cent 
P205. 

Both  substances  are  orthorhombic  (bipyramidal  class),  with  an  axial 
ratio  approximating  .4348  :  1  :  5265.  Crystals  are  rare  and  not  well 
developed.  They  are  usually  rough  prisms  bounded  by  00  P  06  (010), 
oP(ooi),  ooP(no),  ooP2(i2o)  and  2P06  (021).  The  minerals  usually 
occur  massive,  or  in  irregular,  rounded  crystals,  with  two  very  dis- 
tinct cleavages. 

Both  minerals  are  transparent  to  translucent,  both  have  a  white 
streak,  and  both  are  vitreous  to  resinous  in  luster.  Their  hardness  is 
about  4.5-5  and  sp.  gr.  about  3.5.  Triphylite  is  greenish  gray  to  blue, 
and  lithiophilite  pink,  yellow  or  brown.  The  refractive  indices  for 
light  brown  lithiophilite  are:  a=  1.676,  £=1.679,  7=1.687;  those  for 
blue  triphylite  are  a  trifle  higher. 

When  heated  in  closed  tubes  both  compounds  are  apt  to  turn  dark. 
They  fuse  at  a  low  temperature  (1.5)  and  color  the  flame  crimson.  In 
the  case  of  triphylite  the  crimson  streak  is  bordered  by  the  green  of  iron. 
Lithiophilite  gives  the  reactions  for  Mn.  Most  specimens  give  reac- 
tions for  all  these  metals — Fe,  Mn  and  Li.  Both  minerals  are  soluble 
in  HC1. 

The  two  minerals  are  distinguished  from  other  compounds  by  their 
reactions  for  phosphorus  and  lithium,  and  from  each  other  by  the  reac- 
tions for  Fe  and  Mn. 

Occurrence. — They  usually  occur  as  primary  constituents  of  coarse 
granite  veins.  They  are  associated  with  beryl,  tourmaline  and  other 
pneumatolytic  minerals  and  with  secondary  phosphates,  which  are 
presumably  weathering  products  of  the  primary  phosphates. 

Localities. — Both  minerals  occur  at  a  number  of  points  associated 
with  other  lithium  compounds,  especially  spodumene  (p.  378).  In  this 
country  triphylite  has  been  found  at  Peru,  Maine;  Grafton,  New 
Hampshire,  and  Norwich,  Massachusetts;  lithiophilite  at  Branchville, 
Connecticut,  and  at  Norway,  Maine. 

Neither  of  the  minerals  possesses  a  commercial  value  at  present. 


PHOSPHATES,  ARSENATES  AND  VANADATES     263 


Beryllonite  (NaBeP04) 

Beryllonite  is  a  comparatively  rare  mineral  occurring  at  only  a  few 
places  and  always  in  crystals  or  in  crystalline  grains. 

Its  composition  is  24.4  per  cent  Na20,  19.7  per  cent  BeO  and  55.9 
per  cent  P2O5. 

Its  crystals  are  orthorhombic  (bipyramidal  class),  with  an  axial 
ratio  .5724  :  1  :  .5490.  They  are  short  pyramidal  or  tabular  in  habit, 
often  exhibiting  a  pseudohexagonal  symmetry.  Most  crystals  are 
highly  modified  with  oP(ooi),  ooPw(ioo),  00  P  66  (010),  P  66(101) 
and  2P2(i2i),  the  principal  forms.  Twins  are  common,  with  00  P(no) 
the  twinning  plane.    The  crystal  faces  are  frequently  strongly  etched. 

The  mineral  is  white  to  pale  yellow.  It  has  a  vitreous  luster, 
except  on  oP(ooi),  where  the  luster  is  sometimes  pearly.  It  possesses 
four  cleavages,  of  which  the  most  perfect  is  parallel  to  oP(ooi).  That 
parallel  to  ooPw  (100)  is  distinct,  but  the  others  are  indistinct.  Its 
hardness  is  5.5-6  and  its  density  2.845.  ^s  fracture  is  conchoidal. 
Crystals  often  contain  numerous  inclusions  of  water  and  liquid  CO2 
}  arranged  in  lines  parallel  to  c.  Its  refractive  indices  for  yellow  light 
are:  0=1.5520,13=1.5579,7=1.5608. 

Beryllonite  decrepitates  and  fuses  in  the  blowpipe  flame  to  a  cloudy 
glass,  at  the  same  time  imparting  to  the  flame  a  yellow  color.  It  is 
slowly  soluble  in  HC1,  and  gives  the  phosphorus  reaction  with  mag- 
nesium. 

It  is  distinguished  from  most  other  colorless  transparent  minerals 
by  the  reaction  for  phosphorus;  from  other  colorless  phosphates  by  its 
crystallization  and  the  sodium  flame  test. 

Occurrence  and  Localities. — The  best  known  occurrence  of  beryllo- 
nite in  the  United  States  is  Stoneham,  Maine,  where  it  is  found  in  the 
debris  of  a  pegmatite  dike  associated  with  apatite  (p.  266),  beryl  (p.  359), 
and  other  common  constituents  of  pegmatites.  It  originally  existed 
implanted  on  the  walls  of  cavities  in  the  pegmatite  and  was  apparently 
the  result  of  pneumatolytic  processes. 

Use. — The  mineral  is  used  to  some  extent  as  a  gem  stone. 

Monazite  ((Ce  Di  La)P04) 

Monazite  is  the  principal  source  of  certain  rare  earths  that  are  used 
in  manufacturing  gas  mantles.  Although  it  occurs  as  small  grains  and 
crystals  in  certain  granites  it  is  found  in  commercial  quantities  only  in 
the  sands  of  streams. 


264  DESCRIPTIVE  MINERALOGY 

The  mineral  is  a  phosphate  of  the  metals  cerium,  lanthanum,  praseo- 
didymium  and  neodidymium  in  most  cases  combined  with  the  silicate  of 
thorium.     Its  composition  may  be  represented  by  the  formula 

*((Ce  •  La  •  Di)P04)+y(ThSi04), 

in  which  the  proportion  of  the  second  constituent  varies  from  a  trace  to 
an  amount  yielding  20  per  cent  ThCfe.  Since  this  is  not  constant  in 
quantity  it  is  not  to  be  regarded  as  an  essential  portion  of  the  com- 
pound. It  is  probable  that  in  monazite  we  have  to  do  with  a  solid 
solution  of  cerium  and  thorium  phosphates,  thorium  silicate  and  oxides 
of  the  rare  metals. 

Monazite  is  monoclinic  with  a  :  b  :  £=.9693  :  1  :  .9255  and  j5= 
760  20'.  Crystals  are  usually  prismatic  with  the  pinacoids  00  P  *  (100), 
00 Poo  (010),  the  prism  ooP(no),  the  two  domes  —  P  60(101)  and 
+P66(ioT)   and  the  pyramids   —  P(ni)  and  +P(nT).      They    are 

often   flattened  parallel  to  the  orthopinacoid 
(Fig.  150).    The  angle  iioAiTo=86°  34'. 

Their  cleavage  is  perfect  parallel  to  oP. 
The  color  of  the  mineral  is  gray,  yellow,  red- 
dish, brown  or  green.  It  is  usually  transpar- 
ent or  translucent  and  sometimes  opaque.  It 
is  brittle,  has  a  white  streak,  and  a  resinous 
luster.  Its  hardness  is  5-5.5  and  its  sp.  gr. 
Fig.  150.— Monazite  Crys-  4.7-5.3,  varying  with  the  proportion  of  thorium 
tal  with oop do,  100(a);  present>     j^    refractive    indices    for    yellow 

00P,    no    (*w);      00  P2,    ...  00 

'  „w  ,.v    light  are:  a=  1.7038,  7=1.8452. 

120  («);  00  P  00  ,  010  (6);      °  m  §/°   '   '  ^D 

-Poo,  101  («/)•  +P00,         The  mineral  is  infusible.     Before  the  blow- 
10T  (x)  and  P,  nT  (v).      pipe  it  turns  gray,  and  when  moistened  with 

H2SO4  it  colors  the  flame  bluish  green.  It  is 
difficultly  soluble  in  HC1  and  HNO3.  Most  specimens  are  strongly 
radioactive. 

Synthesis. — Crystals  of  monazite  have  not  been  prepared,  but  crys- 
tals of  cerium  phosphate  similar  to  those  of  monazite  have  been  made 
by  heating  to  redness  a  mixture  of  cerium  phosphate  and  cerium  chloride. 
Occurrence  and  Origin. — Monazite  occurs  as  the  constituent  of  cer- 
tain granites  and  granitic  schists  in  small  crystals  scattered  among  the 
other  components.  In  this  form  it  is  a  separation  from  the  granitic 
magma.  When  the  granites  are  broken  down  to  sand  by  weathering 
the  monazite  is  freed  and  because  of  its  specific  gravity  it  concentrates 
in  stream  channels. 

Localities. — Although  the  mineral  is  fairly  widespread  in  the  rocks, 


PHOSPHATES,  ARSENATES  AND   VANADATES     265 


it  is  concentrated  into  commercial  deposits  at  only  a  few  places.  The 
most  important  of  these  are  in  southeastern  Brazil,  in  Norway,  and  in  a 
belt  20  to  30  miles  wide  and  150  miles  long  extending  along  the  east  side 
of  the  Appalachian  Mountains  from  North  Carolina  into  South  Carolina. 

The  mineral  has  also  been  reported  from  many  points  in  ten  coun- 
ties in  Idaho.  Near  Centerville  it  may  be  in  sufficient  quantity  to  be 
of  commercial  importance. 

Preparation. — Monazite  is  separated  from  the  valueless  sand  in 
which  it  is  found,  by  washing,  and  the  residues  thus  resulting  are  further 
concentrated  by  a  magnetic  process.  The  commercial  concentrates 
produced  in  this  way  usually  contain  from  3  to  9  per  cent  TI1O2,  and 
their  price  varies  accordingly. 

.  Production  and  Uses. — Monazite  is  the  chief  source  of  thorium  oxide 
used  in  the  manufacture  of  incandescent  gas  mantles.  Formerly  it  was 
produced  in  large  quantity  in  the  Carolinas,  the  production  in  1909 
amounting  to  542,000  lb.,  valued  at  $65,032,  and  in  1905  to 
1,352,418  lb.,  valued  at  $163,908.  All  of  this  was  manufactured  into 
the  nitrate  of  thorium  in  this  country  and  the  amount  made  was 
not  sufficient  to  meet  the  domestic  demand.  Consequently,  large  quan- 
tities of  the  nitrate  were  imported.  In  1910-11  mining  of  the  mineral 
in  the  Carolinas  ceased  and  all  the  monazite  needed  has  been  imported 
since  then.  The  imports  of  thorium  nitrate  for  191 2  were  117,485  lb., 
valued  at  $225,386  and  of  monazite,  an  amount  valued  at  $47,334. 


a^ 


Xenotime  (YP04) 

Xenotime,  though  essentially  an  yttrium  phosphate,  usually  contains 
erbium  and  in  some  cases  cerium. 
It  occurs  in  tetragonal  crystals 
and  in  rolled  grains.  Its  axial 
ratio  is  1  :  .6177  and  the  angle 
111A1T1  =55°  30'.  Its  crystals 
are  octahedral  or  prismatic  and 
are  bounded  by  00  P(i  10),  P(i  11), 
and  in  some  cases  by  00  P  00  (100) 
and  2P  00  (201)  (Fig.  151).  Their 
cleavage    is    perfect  parallel   to  Fig.  151.— Xenotime  Crystals  with  °op,  iXO 

■n/        \     »m.        •  1  •    u  (OT)>  P-  I"  W,  and   00  Poo;  ioo  (a). 

ooP(no).  The  mineral  is  brown,  w'  '        w 

pink,  gray  or  yellow.  Its  streak  is  a  pale  shade  of  the  same  color. 
It  is  opaque  and  brittle.  Its  luster  is  vitreous  or  resinous;  its  hardness 
4-5  and  specific  gravity  4.5.    Its  indices  of  refraction  are:   e=i.8i, 

tt=1.72. 


266  DESCRIPTIVE  MINERALOGY 

Xenotime  is  infusible,  insoluble  in  acids  and  with  difficulty  soluble 
in  molten  microcosmic  salt.  It  is  distinguished  from  zircon  by  its 
cleavage  and  inferior  hardness. 

A  variety  of  xenotime  containing  a  small  percentage  of  sulphates  is 
known  as  hussakite. 

The  mineral  occurs  in  pegmatite  veins,  in  granites  and  in  the  sands 
of  streams.  It  is  found  in  pegmatite  veins  at  Hittero,  Moss,  and  other 
places  in  Norway;  at  Ytterby,  Sweden;  in  the  granites  of  Minas  Geraes, 
Brazil,  and  in  the  gold  washings  at  Clarksville,  Georgia,  and  many  places 
in  North  Carolina,  and  in  pegmatite  veins  in  Alexander  County  in  the 
same  State. 

APATITE  GROUP 

The  apatite  group  consists  of  a  number  of  phosphates,  arsenates  and 
vanadates  in  which  fluorine  or  chlorine  takes  the  place  of  the  hydroxyl 
in  basic  compounds.  Thus,  jluorapatite  is  Ca4(CaF)(P04)3  and  chlor- 
apatite  Ca4(CaCl)(P04)3-  The  group  contains  a  number  of  important 
minerals,  of  which  apatite  is  by  far  the  most  valuable.  These  minerals 
are  isomorphous,  all  crystallizing  in  the  hemihedral  division  of  the  hex- 
agonal system  (hexagonal  bipyramidal  class).  The  names,  composi- 
tions and  axial  ratios  of  the  most  important  are  as  follows: 

Fluor  apatite  Ca4(CaF)(P04)3  a:  c=i  :  .7346 

Chlorapatite  Ca4(CaCl)(P04)3  a  :  c=i  :  .7346+ 

Pyromorphite  Pbi(PbCl)(P04)3  a  :  c=i  :  .7293 

Mimetite  Pbi(PbCl)(As04)3  a  :  c=i  :  .7315 

Vanadinite  Pb4(PbCl)(V04)3  a  :  c=i  :  .7122 

Apatite  (Ca4(Ca(F-Cl))(P04)3) 

Although  fluorapatite  and  chlorapatite  are  distinct  compounds  with 
slightly  different  properties,  nevertheless,  because  of  the  difficulty  of 
discriminating  between  them  without  analyses,  the  name  apatite  is 
commonly  applied  to  both.  This  is  justified  because  of  the  fact  that  the 
two  compounds  are  completely  isomorphous,  and  the  mineral  as  it 
usually  occurs  is  a  mixture  of  both.  The  ideal  molecules  comprising 
the  two  varieties  of  apatite  have  the  following  compositions: 

Fluorapatite     CaO=55-5,   F=3.8,  P20s  =  42.3 
Chlorapatite     CaO=53-8,  Cl  =  6.8,  P20s  =  4i.o 

Apatite  is  found  in  well  defined  crystals,  sometimes  very  large. 
These  have  a  holohedral  habit,  but  etch  figures  on  their  basal  planes 


PHOSPHATES,  ARSENATES  AND  VANADATES     267 

reveal  the  grade  of  symmetry  of  pyramidal  hemihedrism.  The  min- 
eral occurs  also  massive,  in  granular  and  fibrous  aggregates  and  less 
commonly  in  globular  forms  and  as  crusts. 

The  crystals  are  usually  columnar  or  tabular,  with  the  hexagonal 
prism  or  pyramid  well  developed.  Although  in  some  cases  highly 
modified,  most  crystals  contain  only  the  oo  P(iolo),  P(ioTi)  and  oP(oooi) 
planes  prominent,  though  £P(iol2)  and  2P2(ii2i)  are  not  uncommon  as 
small  faces  (Figs.  152  and  153).  Their  cleavage  is  indistinct,  and  their 
fracture  often  conchoidal. 

Apatite  may  possess  almost  any  color.  In  a  few  cases  the  mineral  is 
colorless  or  amethystine  and  transparent,  but  in  most  cases  it  is  trans- 
lucent or  opaque  and  white,  green,  bluish,  brown  or  red.    Its  streak  is 


Fig.  152. 


Fig.  153. 


Fig.  152. — Apatite  Crystals  with   00  P,   1010  (w);   P,  1011   (.r);    oP,  oooi  (c);    JP, 

1012  (r)  and  00  P2,  1120  (a). 

Fig.  153. — Apatite  Crystal  with  m,  x,  r  and  c  as  in  Fig.  152  and  2P,  2021  (y);  4PJ, 
1341  00;  3P}>  i23i  GO;   2P2,  1121  (5);   P2,  1122  (»)  and  00 p|,  1230  (A). 


white  and  its  luster  vitreous  to  resinous.  Its  hardness  is  4.5-5  an^  sp. 
gr.  between  3.09  and  3.39.  The  refractive  indices  of  fluorapatite  for 
yellow  light  are:  «=  1.6335,  €=1.6316  and  of  chlorapatite,  o>=  1.6667. 
Many  specimens  are  distinctly  phosphorescent.  Nearly  all  fluoresce  in 
yellowish  green  tints,  and  all  are  thermo-electric. 

Apatite  fuses  with  difficulty,  tinging  the  flame  reddish  yellow.  The 
chlorapatite  melts  at  15300  and  the  fluorine  variety  at  16500.  When 
moistened  with  H2SO4  all  varieties  color  the  flame  pale  bluish  green, 
due  to  the  phosphoric  acid.  Specimens  containing  chlorine  give  the 
brilliant  blue  color  to  the  flame  when  fused  in  a  bead  of  microcosmic 
salt  that  has  been  saturated  with  copper  oxide.  Specimens  containing 
fluorine  etch  glass  when  fused  with  this  salt  in  an  open  glass  tube. 
The  mineral  also  yields  phosphine  when  ignited  with  magnesium,  and 
it  dissolves  in  HC1  and  HNO3. 


268  DESCRIPTIVE  MINERALOGY 

Apatite  is  much  softer  than  beryl  (p.  359),  which  it  closely  resembles 
in  appearance.  It  is  distinguished  from  calciU  by  lack  of  effervescence 
with  acids  and   from  other  compounds  by  the  phosphorus   reaction. 

The  varieties  of  the  mineral  recognized  by  distinct  names  are: 

Ordinary  apatite,  crystals  or  granular  masses. 

Manganapatite,  in  which  manganese  partly  replaces  the  Ca  of  ordi- 
nary apatite.    This  is  dark  bluish  green. 

Fibrous,  concretionary  apatite.    Known  also  as  phosphorite. 

Osteolite.    The  earthy  variety. 

Phosphate  rock.  A  mixture  of  apatite,  phosphorite,  several  hydrous 
carbonates  and  phosphates  of  calcium,  and  fragments  of  bone  and 
teeth.  It  is  more  properly  a  rock  with  a  brecciated  and  concretionary 
structure.  The  composition  of  typical  deposits  is  represented  by  the 
following  analysis  of  hard  rock  phosphate  from  South  Carolina: 

CaO      P2O5    C02  Fe203  AI2O3    MgO  Insol.  Undet.    H20     Moist. 
50.08      38.84    .65      .96      3.07       .30     .49      2.46      2.96        .07 

Guano  is  a  mixture  of  various  phosphates,  both  hydrous  and  an- 
hydrous, calcite  and  a  number  of  other  compounds.  It  is  rather  a  rock 
than  a  mineral,  as  it  has  no  definite  composition. 

Syntheses. — Crystals  of  fluorapatite  have  been  made  by  fusing 
sodium  phosphate  with  CaF2  and  by  heating  calcium  phosphate  with  a 
mixture  of  KF  and  KC1. 

Origin. — The  crystallized  apatite  was  formed  by  direct  separation 
from  igneous  rock  magmas  and  by  pneumatolytic  action  upon  limestone. 
The  phosphorite  variety  and  the  phosphate  in  phosphate  rock  were 
probably  produced  by  the  solution  of  calcium  phosphate  and  its  later 
deposition  from  solution — the  original  phosphate  having  been  furnished 
in  many  cases  by  the  shells  of  mollusca,  and  by  the  action  of  phosphoric 
acid  produced  by  the  decay  of  organisms  upon  limestone.  In  many 
cases  phosphorite  accumulated  as  a  residual  deposit  in  consequence  of 
the  solution  of  the  calcite  and  dolomite  from  phosphatic  limestone, 
leaving  the  less  soluble  phosphate  as  a  mantle  on  the  surface. 

Occurrence. — The  mineral  occurs  in  microscopic  crystals- as  a  com- 
ponent of  many  rocks,  as  large  crystals  in  metamorphosed  limestones, 
as  a  component  of  many  coarse-grained  veins,  especially  those  composed 
of  coarse  granite  and  those  in  which  cassiterite,  magnetite,  tourmaline, 
and  other  pneumatolytic  minerals  are  found.  At  a  number  of  places 
aggregates  of  apatite  and  magnetite  or  ilmenite  occur  in  such  large 
masses  as  to  be  worthy  of  being  called  rocks.  An  impure  apatite  in 
concretionary  and  fibrous  forms  also  occurs  in  thin  beds  covering  large 


PHOSPHATES,  ARSENATES  AND  VANADATES     269 

areas.  It  is  often  mixed  with  other  phosphates,  with  the  bones  and 
teeth  of  animals  and  with  other  impurities.  This  is  the  well  known 
phosphate  rock  or  phosphorite. 

Localities. — Crystallized  apatite  is  so  widely  spread  that  it  is  useless 
to  mention  its  occurrences.  It  is  mined  at  Kragero  and  near  Bamle, 
in  Norway;  at  various  points  in  Ottawa  County  in  Quebec,  and  in 
Frontenac,  Lanark  and  Leeds  Counties  in  Ontario;  and  at  Mineville, 
New  York.  Rock  phosphate  is  found  in  extensive  beds  on  the  west 
side  of  the  peninsula  of  Florida,  in  South  Carolina,  North  Carolina, 
Alabama,  Tennessee,  Wyoming,  Idaho,  Utah  and  Arkansas.  A  mixture 
of  apatite  and  ilmenite  {nelsonite) ,  occurs  as  dikes  in  Nelson  and 
Roanoke  Counties,  Virginia. 

Uses. — The  principal  use  of  apatite  and  phosphate  rock  is  in  the 
manufacture  of  fertilizers.  The  rock  (or  crushed  apatite)  is  treated 
with  H2SO4  to  make  an  acid  phosphate  which  is  soluble  in  water.  Am- 
monia or  potash,  or  both,  are  added  to  the  mass  and  the  compound  is 
sold  as  a  superphosphate.  The  purest  varieties  are  treated  with  H2SO4 
in  sufficient  quantity  to  entirely  decompose  them,  CaS04  and  H3PO4 
being  formed.  The  latter  is  drawn  off  and  mixed  with  additional  high- 
grade  rock  and  the  mixture  is  known  as  concentrated  phosphate.  Super- 
phosphates are  manufactured  in  large  quantities  in  the  United  States 
and  the  concentrated  phosphates  in  Europe.  Unfortunately,  for  the 
latter  use  the  best  grades  of  apatite  or  rock  phosphate  are  required,  and 
consequently  the  best  grades  of  rock  produced  in  the  United  States  are 
exported  and  thus  lost  to  American  farmers. 

Production. — The  world's  production  of  apatite  and  phosphate  rock 
during  191 2  was  as  follows: 

United  States 3,020,905  tons,  valued  at  $11,675,774 

Tunis 2,050,200  tons,  valued  at  7,500,000 

Christmas  Island. .  159,459  tons,  valued  at  2,024,036 

France ...  313,151  tons,  valued  at  1,169,400 

Algeria 207,111  tons,  valued  at  759,455 

Belgium 203,110  tons,  valued  at  316,703 

Other  countries 65,000  tons,  valued  at  280,000 

For  the  United  States  production  of  191 2  the  statistics  are: 

Florida 2,407,000  tons,  valued  at  $9,461,000 

Tennessee 423,300  tons,  valued  at    1,640,500 

South  Carolina.. .  .  131,500  tons,  valued  at       524,700 

Other  States 11,600  tons,  valued  at         49,200 


270  DESCRIPTIVE  MINERALOGY 

The  total  production  was  3,020,905  tons,  valued  at  $11,675,774.00, 
of  which  1,206,520  tons,  valued  at  $8,996,456.00  were  exported.  Par- 
tially offsetting  this,  there  were  imported  guano,  apatite  and  other  phos- 
phates to  the  value  of  about  $2,000  000. 

Pyromorphite  (Pb4(PbCl)(P04)3) 

In  composition  pyromorphite  is  PbO,  82.2  per  cent,  P2O5,  15.7  per 
cent  and  CI,  2.6  per  cent,  but  there  are  usually  present  also  CaO  and 
AS2O5. 

The  mineral  is  completely  isomorphous  with  apatite.  Its  crystals 
are  smaller  and  simpler  than  those  of  apatite,  but  they  have  the  same 
habit.  Their  axial  ratio  is  a  :  c=i  :  .7293.  This  increases  to  1  :  .7354 
in  varieties  containing  calcium. 

Crystals  are  often  rounded  into  barrel-shaped  forms,  and  frequently 
are  mere  skeletons.  Tapering  groups  of  slender  crystals  in  parallel 
growths  are  also  common.  Their  cleavage  is  parallel  to  the  00  P(no) 
faces,  and  their  fracture  is  feebly  conchoidal.  The  mineral  also  occurs 
in  globular,  granular  and  fibrous  masses. 

Pyromorphite  is  translucent.  It  is  brittle,  has  a  hardness  of  3.5-4 
and  a  density  of  about  7.  Its  luster  is  resinous  and  color  usually  green, 
yellow,  brown  or  orange.  Some  varieties  are  gray  or  milk-white.  Its 
streak  is  white.  Its  refractive  indices  for  yellow  light  are:  «=  2.0614, 
€=  2.0494.     The  mineral  is  distinctly  thermo-electric. 

When  heated  in  the  closed  tube  pyromorphite  gives  a  white  subli- 
mate of  lead  chloride.  It  fuses  easily,  coloring  the  flame  bluish  green. 
When  heated  on  charcoal  it  melts  to  a  globule,  which  crystallizes  on 
cooling  and  yields  a  coating  which  is  yellow  (PbO)  near  the  assay  and 
white  (PbCfe),  at  a  greater  distance  from  it.  When  fused  with  Na2C03 
on  charcoal  a  globule  of  lead  results.  The  mineral  also  gives  the  CI  and 
P  reactions.    The  mineral  is  soluble  in  HNO3. 

Pyromorphite  is  recognized  by  its  form,  high  specific  gravity  and  its 
action  when  heated  on  charcoal. 

Synthesis. — Crystals  have  been  obtained  by  fusing  sodium  phosphate 
with  PbCl2. 

Occurrence. — The  mineral  occurs  principally  in  veins  with  other  lead 
ores,  especially  in  the  zone  of  weathering.  It  also  exists  in  pseudomorphs 
after  galena. 

Localities. — It  is  found  in  all  lead-producing  regions,  especially  in 
the  upper  portions  of  veins.  It  occurs  in  particularly  good  specimens 
at  Pribram,  Bohemia;  at  Ems,  in  Nassau;  in  Cornwall,  Devon,  Derby- 


PHOSPHATES,  ARSENATES  AND  VANADATES     271 

shire  and  Cumberland,  England;  at  Phoenix ville,  Pennsylvania,  and 
at  various  other  points  in  the  Appalachian  region. 

Uses. — Pyromorphite  alone  possesses  no  commercial  value,  but  it 
is  mined  with  other  compounds  of  lead  as  an  ore  of  this  metal. 

Mimetite  (Pb4(PbCl)(As04)3) 

Mimetite,  or  mimetesite,  resembles  pyromorphite  in  its  crystals  and 
general  appearance,  and  many  of  its  properties.  Its  color,  however,  is 
lighter  and  its  density  slightly  greater.  It  occurs  in  crystals,  in  fila- 
ments, and  in  concretionary  masses  and  crusts.  Its  axial  ratio  is 
i  :  .7315  and  its  refractive  indices  for  yellow  light  are:  £0=2.1443,  e 
=  2.1286. 

The  formula  for  mimetite  demands  74.9  per  cent  PbO,  23.2  per  cent 
AS2O5  and  2.4  CI.  Usually  a  portion  of  the  lead  is  replaced  by  CaO  and 
a  portion  of  the  As  by  P. 

Mimetite  fuses  more  easily  than  pyromorphite.  It  differs  from  this 
mineral  in  yielding  arsenical  fumes  when  heated  on  charcoal.  More- 
over, when  heated  in  a  closed  tube  with  a  fragment  of  charcoal  it  coats 
the  walls  of  the  tube  with  metallic  arsenic. 

Occurreftce  and  Localities. — It  occurs  with  other  lead  minerals  in 
veins,  usually  coating  them  either  as  crusts  or  as  a  series  of  small  crys- 
tals. It  is  found  at  Phoenix  ville,  Pennsylvania;  in  Cornwall,  England; 
at  Johanngeorgenstadt,  in  Germany;  at  Nerchinsk,  Siberia;  at  Lang- 
ban,  in  Sweden,  and  at  a  number  of  other  places.  It  is,  however,  not 
as  common  as  the  corresponding  phosphorus  compound. 

Uses. — It  is  mined  with  other  compounds  as  an  ore  of  lead. 

Vanadinite  (Pb4(PbCl)(V04))3 

Vanadinite  is  the  most  widely  distributed  of  all  the  vanadium  min- 
erals. It  usually  occurs  in  small  bright  red  prismatic  crystals  implanted 
on  other  minerals,  or  on  the  walls  of  crevices  in  rocks.  It  is  one  of  the 
sources  of  vanadium. 

Its  theoretical  composition  is  as  follows:  PbO  =78.7  per  cent, 
Vo05=i94  per  cent  and  CI  =2.5  per  cent,  but  phosphorus  and  arsenic 
are  often  also  present.  When  arsenic  and  vanadium  are  present  in 
nearly  equal  quantities  the  mineral  is  known  as  endlichitc. 

Its  crystals  are  hexagonal  prisms  and  pyramids  bounded  by 
ooP(ioTo),  oP(oooi),  ooP2(ii2o),  P(ioTi)  and  other  forms,  with  an 
axial  ratio  1  :  .7122  (Fig.  154).    Often  the  crystals  have  hollow  faces 


272  DESCRIPTIVE  MINERALOGY 

(Fig,  155)-  Frequently  they  are  grouped  into  pyramids  like  those  of 
pyromorphite.    The  mineral  occurs  also  in  globules  and  crusts. 

Vanadinite  is  brittle,  has  a  hardness  of  about  3  and  a  specific  gravity 
of  about  7.  Its  fracture  is  conchoidal.  Its  luster  is  adamantine  or 
resinous  and  its  color  ruby  red,  brownish  yellow  or  reddish  brown. 
Its  streak  is  white  or  light  yellow.  The  mineral  is  translucent 
or  opaque.  Its  refractive  indices  for  yellow  light  are:  (0=2.354, 
4=2.299. 

In  the  closed  tube  vanadinite  decrepitates.  It  fuses  easily  on  char- 
coal to  a  black  lustrous  mass  which  is  reduced  on  being  further  heated 
in  the  reducing  flame  to  a  globule  of  lead.  A  white  sublimate  of  PbCfe 
also  coats  the  charcoal.    The  mineral,  moreover,  gives  the  flame  test 


f^\ 


\^sJJ 


—Vanadinite  Crystal  with  °°P,  1010  (in);  oP,  1 


m- 


Fig.  155. — Skeleton  Crystal  of  Vanadinite. 

for  chlorine  with  copper.  After  complete  oxidation  of  the  lead  by  heat- 
ing in  the  oxidizing  flame  on  charcoal  the  residue  gives  an  emerald-green 
bead  in  the  reducing  flame  with  microcosmic  salt  and  this  turns  to  a 
light  yellow  in  the  oxidizing  flame.  The  mineral  is  soluble  in  hydro- 
chloric acid.  If  to  the  solution  a  little  hydrogen  peroxide  is  added  it 
will  turn  brown.  The  addition  of  metallic  tin  to  this  will  cause  it  to 
turn  blue,  green  and  lavender  in  succession,  in  consequence  of  the  reduc- 
tion of  the  vanadium  compounds. 

Vanadinite  is  easily  distinguished  from  most  other  minerals  by  its 
color.  It  is  distinguished  from  other  compounds  of  the  same  color  by 
its  crystallization  and  by  the  reactions  for  vanadium. 

Occurrence. — Vanadinite  occurs  principally  in  regions  of  volcanic 
rocks.     It  is  probably  a  result  of  pneumatolytic  processes. 

Localities. — Crystals  are  found  at  Zimapan,  Mexico;   Wanlockhead, 


PHOSPHATES,  ARSENATES  AND  VANADATES      273 

England;  Undenas,  Sweden;  in  the  Sierra  de  C6rdoba,  Argentine,  and  in 
the  mining  districts  of  Arizona  and  New  Mexico. 

Uses. — Vanadinite  is  an  important  source  of  vanadium,  which  is 
employed  in  the  manufacture  of  certain  grades  of  steel  and  bronze. 
Its  compounds  are,  moreover,  used  as  pigments  and  mordants.  Most 
of  the  vanadium  compounds  produced  in  this  country  are  obtained  from 
other  vanadium  minerals,  among  them  patronite — a  mixture,  of  which 
the  principal  component  is  a  sulphide  (VS4) — and  carnotite  (p.  290), 
but  vanadinite  has  been  used  abroad  and  also  to  a  small  extent  in  the 
United  States. 

WAGNERITE  GROUP 

This  group,  in  chemical  composition,  is  analogous  to  the  apatite 
group.  It  includes  a  number  of  phosphates  and  arsenates  containing  a 
fluoride  radical.  The  group  is  monoclinic  (prismatic  class),  with  an 
axial  ratio  which  is  approximately  1.9  :  1  :  1.5,  with  £=71°  50'.  None 
of  its  members  are  important.  The  two  most  common  ones  are  wag- 
nerite  (Mg(MgF)P04),  and  triplite  (Fe-Mn)  ((Fe-Mn)F)P04. 

Wagnerite  occurs  in  massive  forms  and  in  large  rough  crystals,  with 
imperfect  cleavages  parallel  to  00  P  <x>  (100)  and  00  P(no).  Its  crystals 
have  an  axial  ratio  of  1.9145  :  1  :  1.5059  with  /3=7i°  53'.  They  are 
often  very  complex.  The  mineral  is  brittle.  Its  fracture  is  uneven. 
Its  hardness  is  5.5  and  density  3.09.  Its  color  is  yellow,  gray,  pink  or 
green.  It  is  vitreous,  translucent  and  has  a  white  streak.  Its  refractive 
indices  are:  a=  1.569,  0=  1.570,  7  =  1.582.  It  fuses  to  a  greenish  gray 
glass  and  gives  the  usual  reactions  for  fluorine  and  phosphoric  acid.  It 
is  soluble  in  HC1  and  HNO3,  and  heated  with  H2SO4  it  yields  hydro- 
fluoric acid.  It  occurs  in  good  crystals  near  Werfen,  Austria,  and  in 
coarse  crystals  near  Bamle,  Norway. 

Triplite  is  an  isomorphous  mixture  of  Fe(FeF)PC>4  and  Mn(MnF)PC>4. 
It  usually  occurs  massive,  but  is  found  in  a  few  places  in  rough  crystals. 
The  mineral  is  dark  brown  or  nearly  black,  is  translucent  to  opaque, 
and  has  a  yellowish  gray  or  brown  streak.  It  possesses  two  unequal 
cleavages  perpendicular  to  one  another  and  a  weakly  conchoidal  frac- 
ture. Its  hardness  is  4-5.5  and  specific  gravity  about  3.9.  Its  luster  is 
resinous.     Its  intermediate  refractive  index  is  1.660. 

Before  the  blowpipe  triplite  fuses  easily  (1.5)  to  a  black  magnetic 
globule.  It  reacts  for  Mn,  Fe,  F,  and  P2O5.  It  is  soluble  in  HC1  and 
evolves  hydrofluoric  acid  with  H2SO4.     It  is  found  in  coarse  granite 


274  DESCRIPTIVE  MINERALOGY 

veins  at  Limoges,  France;  Helsingfors,  Finland;  Stoneham,  Maine; 
and  Branchville,  Connecticut.  In  all  of  its  occurrences  it  appears  to 
be  pneumatolytic. 

BASIC  PHOSPHATES  AND  ARSENATES 

The  basic  phosphates  are  those  in  which  there  is  more  metal  present 
than  sufficient  to  replace  the  three  hydrogen  atoms  in  the  normal  acid, 
H3PO4.  This  is  due  to  the  replacement  of  one  or  more  of  the  hydrogen 
atoms  by  a  group  of  atoms  consisting  of  a  metal  and  hydroxyl  (OH). 
All  yield  water  when  heated  in  the  closed  tube. 

The  principal  basic  phosphates  are  amblygonite,  a  source  of  lithium 
compounds,  dufrenite  and  lazulite,  neither  of  which  is  of  economic  im- 
portance, and  libethenite^  a  copper  compound  which  occurs  in  compara- 
tively small  quantities  with  other  copper  ores,  and  is  mined  with 
them. 

Olivenite  is  a  basic  copper  arsenate  corresponding  to  the  phosphate 
libethenite. 

Amblygonite  (Li(Al(F  OH))P04) 

Amblygonite  is  an  isomorphous  mixture  of  the  two  compounds 
(AlF)LiP04  and  (AlOHQLiPQi.    It  is  an  important  source  of  lithium. 

The  composition  of  the  fluorine  molecule  is  Al20a  =  34.4  per  cent, 
Li02=io.i  per  cent  and  P20s  =  47.9  per  cent,  making  a  total  of  105.3 
per  cent  from  which  deducting  5.3  per  cent  (0=  2F),  leaves  100.  Nearly 
always  a  portion  of  the  F  is  replaced  by  OH  and  a  part  of  the  Li 
by  Na.  The  pure  Na(A10H)PC>4  is  known  Sisfremonttie,  and  the  pure 
Li(AlOH)P04  as  montcbrasite. 

The  analysis  of  a  specimen  from  Paia,  California,  gave: 


P«Os 

AltOa 

FeaOi 

MnO 

MgO 

Li,0 

NajO 

H.O 

0-P    Total 

48.83 

33.70 

.12 

.09 

.31 

9.88 

.14 

595 

2.29  =»ioi.3i  —  .96     —     100.45 

The  mineral  forms  large,  ill-defined  triclinic  crystals  (Fig.  156),  and 
compact  masses  with  a  columnar  cleavage.  Crystals  are  very  rare,  and 
are  poorly  developed.  Their  axial  ratio  is  .7334  :  1  :  .7633.  The 
cleavage  pieces  often  show  polysynthetic  twinning  lamellae  parallel  to 
'P'  00  (101)  and  /Py  00  (Toi). 

The  cleavage  of  the  mineral  is  perfect  parallel  to  oP(ooi).  Its 
fracture  is  uneven.  It  is  brittle,  has  a  hardness  of  6  and  a  density  of 
3.03.  Its  color  is  white,  gray,  or  a  very  light  tint  of  blue,  pink  or 
yellow.     Its  luster  is  vitreous,  except  on  oP  where  it  is  pearly.     Its 


PHOSPHATES,  ARSENATES  AND  VANADATES     275 


ioo  (a);  oP,  ooi  (c); 
oo 'P,  ilo  (M);oop], 
no  (m);  oo^,  1 20 
(z) ;  /P/  00  ,  Toi  (A) 
and  a'P  00  ,  021  (e). 


streak  is  white  and  it  is  translucent.    Its  refractive  indices  for  yellow 
light  are:  a=  1.579,  £=1.593,  7=1.597. 

In  the  closed  tube  at  high  temperature  it  yields  water  which  reacts 
acid  and  corrodes  glass.  It  fuses  easily  to  an 
opaque  white  enamel.  It  colors  the  flame  red 
with  a  slight  fringe  of  green.  When  moistened 
with  H2SO4  it  tinges  the  flame  bluish  green. 
When  finely  powdered  it  dissolves  readily  in 
II2SO4  and  with  dfficulty  in  HC1. 

Amblygonite  resembles  in  appearance  many 
other  minerals,  especially  spodumene  (p.  378), 
and  some  forms  of  barite,  feldspar,  dolomite,  etc.  pIG  IS6— Amblygonite 
From  spodumene  it  is  distinguished  by  the  phos-  Crystal  with  00  p  56 , 
phorus  reaction  and  the  acid  water;  from  the 
others  by  its  easy  fusibility. 

Occurrence. — Amblygonite  is  found  in  granite 
and  in  pegmatite  veins  associated  with  other 
lithium  compounds,  tourmaline,  cassiterite  and 
other  minerals  of  pneumatolytic  origin.  In  all  cases  it  also  is  probably 
a  result  of  pneumatolytic  action  associated  with  the  last  phases  of  granite 
intrusions. 

Localities. — The  mineral  occurs  near  Penig,  in  Saxony;  at  Arendal, 
in  Norway;  at  Montebras,  France;  at  Hebron,  Paris  and  Peru,  Maine; 
at  Branchville,  Conn.;  at  Pala,  in  California;  and  near  Keystone,  in 
the  Black  Hills,  South  Dakota. 

Uses  and  Production. — The  mineral  is  the  principal  source  of  lithium 
compounds  in  the  United  States.  It  is  used  in  the  manufacture  of 
LiC03,  which  is  employed  as  a  medicine,  in  making  mineral  waters,  in 
photography  and  in  pyrotechnics. 

It  has  been  mined  in  South  Dakota  and  in  California  to  the  extent 
of  a  couple  of  thousand  tons,  valued  perhaps  at  $20,000. 

Dufrenite  (Feo(OH)3P04) 

Dufrenite,  or  kraurite,  is  a  basic  iron  phosphate  containing  62  per 
cent  Fe2C>3,  27.5  per  cent  P2O5  and  10.5  per  cent  water.  It  may  be 
regarded  as  a  normal  phosphate  in  which  one  H  atom  of  H3PO4  has  been 
replaced  by  the  Fe(0H)2  group  and  two  by  the  group  Fe(OH),  thus 

HO-Fe=P04-Fe<^' 

It  forms  small  orthorhombic  crystals  with  a  cubic  habit  that  are  rare. 
Their  axial  ratio  is  .3734  :  1  :  .4262.    It  usually  occurs  massive,  in 


276 


DESCRIPTIVE  MINERALOGY 


nodules,  or  in  fibrous  radiating  aggregates.  The  same  substance  is 
believed  to  occur  also  in  the  colloidal  condition  under  the  name  delvauxite. 

The  color  of  dufrenite  varies  from  leek-green  to  dark  green,  which 
alters  on  exposure  to  yellow  and  brown.  It  is  translucent  to  opaque, 
has  a  light  green  streak  and  is  strongly  pleochroic.  Its  hardness  is 
3.5-4  and  specific  gravity  about  3.3. 

In  the  closed  tube  it  yields  water  and  whitens.  It  fuses  easily,  color- 
ing the  flame  bluish  green  and  yielding  a  magnetic  globule.  It  is  sol- 
uble in  HC1  and  in  dilute  H2SOt. 

It  is  recognized  by  its  color  and  the  presence  in  it  of  water,  phos- 
phorus and  iron. 

Localities  and  Origin, — The  mineral  has  been  observed  at  several 
points  in  Europe;  at  Allentown,  New  Jersey,  and  in  Rockbridge  County, 
Virginia.  It  is  thought  to  be  ptoduced  by  the  weathering  of  other  fer- 
ruginous phosphates. 

Lazulite  ((Mg-Fe)(A10H)2(P04)2) 

Lazulite  is  essentially  an  isomorphous  mixture  of  the  two  com- 
pounds Mg(A10H)2(P04)2  and  Fe(AiOH)2(PO.i)2.  There  is  also  fre- 
quently present  in  it  a  little  calcium. 
When  the  proportion  of  the  two 
molecules  present  is  as  2  :  1  the  com- 
position becomes  FeO=7.7;  MgO 
=  8.5;  Al203  =  32.6;  P205=454and 
H20=5.8. 

The  mineral  occurs  in  blue  pyram- 
idal   crystals    that    are    monoclinic 
(prismatic  class),  with  the  axial  ratio 
=  .9750  :  1  :  1.6483  and  £=89°  14'. 
A  u  The  predominant  forms  are  +P(nT), 

Fig.  157.— Lazulite  Crystals.  A  with  —  P(ni)  and  — P  00  (ioi)(Fig.  157^). 
-P,  in  (/>);  +P,  hi  Wand  P«,  The  angle  in  AITI  =  79040,.  Twins 
101  (/).     B  is  the  same  combination   are    not    common>     Those   most  fre. 

twinned  about  00  P  06  (100).  with  oP  .      .         ,  A    .         .      . 

,      x  #.  ,  quently  found  are  twinned  about  c 

(001)  the  composition  face.  M  J 

as   the  twinning   axis    (Fig.    157B). 
It  is  found  also  massive  and  in  granular  aggregates. 

The  cleavage  of  lazulite  is  not  distinct.  Its  fracture  is  uneven.  It 
is  brittle,  has  a  vitreous  luster,  is  translucent  or  opaque,  has  an  azure 
color  and  a  white  streak.  Its  hardness  is  5  or  6  and  its  specific  gravity 
about  3.1.  Translucent  crystals  are  strongly  pleochroic  in  deep  blue 
and  greenish  blue  tints — the  former  when  viewed  along  the  vertical 


B 


PHOSPHATES,  ARSENATES  AND  VANADATES     277 

axis.  Their  indices  of  refraction  for  yellow  light  are:  a=  i .603, 0=  1.632, 
7=1.639. 

In  the  closed  tube  lazulite  swells,  whitens  and  yields  water.  When 
heated  in  the  blowpipe  flame  it  whitens,  falls  to  pieces  and  colors  the 
flame  bluish  green.  The  white  powder  moistened  with  Co(NQs)2  and 
reheated  regains  its  blue  color.  When  moistened  with  H2SO4  and 
heated  in  the  blowpipe  flame  it  imparts  to  it  a  green  blue  color.  It  is 
infusible  and  is  unacted  upon  by  acids. 

Lazulite,  when  massive,  closely  resembles  in  appearance  massive 
forms  of  some  varieties  of  sodalite,  haiiynite  and  lazurite  (p.  333).  The 
latter,  however,  are  soluble  in  HC1.  Moreover,  none  of  them  contains 
phosphorus. 

Occurrence. — The  mineral  occurs  in  quartz  veins  in  sandstones  and 
slates  and  is  usually  a  product  of  metamorphism.  It  is  sometimes,  how- 
ever, found  in  serpentine  rocks,  with  corundum,  in  which  case  it  may  be 
original. 

Localities. — Good  crystals  occur  at  Krieglach,  in  Styria;  at  Horrs- 
joberg,  in  Sweden,  and  in  the  United  States  at  Crowder's  Mountain, 
North  Carolina,  and  on  Graves  Mountain  in  Georgia. 


OLIVENITE  GROUP 

The  olivenite  group  includes  a  number  of  basic  copper,  lead  and 
zinc  compounds  of  the  general  formula  R"2(OH)R/"04  in  which  R" 
=  Cu,  Zn,  Pb  and  R"'=As,  P,  V.  The  group  is 
orthorhombic  (bipyramidal  class),  with  axial  ratios 
approximating  .95  :  1  :  .70.  The  most  important 
members  of  the  group  are  the  two  copper  min- 
erals, olivenite,  Cu(CuOH)  As04  and  libethenite, 
Cu(CuOH)P04. 

Olivenite  occurs  in  fibrous,  globular,  lamellar, 
granular  and  earthy  masses  and   in  prismatic  and 
acicular  crystals  bounded  by  00  P(i  10),  00  P  6b  (100),  FlG-   J58-  —  Olivenite 
ooP66(oio),P56(oii)   andPob(ioi)   (Fig.   158).      CryStal mth °° P °° ' 
Their  axial  ratio  is  .9396  :  1  :  .6726  and  the  angle 
1 10  A 1  To=  86°  26'.    Their  cleavage  is  poor. 

The  mineral  is  some  shade  of  green,  brown, 
yellow  or  grayish  white  and  its  streak  is  olive-green 
in  greenish  varieties.     It  is  transparent  to  opaque,  is  brittle,  has  a 
hardness  =3,  and  a  specific  gravity =4.3.    Its  refractive  indices   for 


100  (a);  00 P,  no 
(w);  00  Poo  ,010(6); 
P  00  ,  01 1  (e )  and 
P  »o  ,  101  (»). 


278  DESCRIPTIVE  MINERALOGY 

yellow  light  are  about  1.83.  Its  luster  is  usually  vitreous.  Fibrous 
varieties  are  sometimes  known  as  wood-copper. 

Olivenite  fuses  easily  (2)  to  a  mass  that  appears  crystalline  on  cooling. 
It  gives  the  usual  reactions  for  H2O,  Cu,  and  As.  It  is  soluble  in  acids 
and  in  ammonia. 

It  is  associated  with  other  copper  compounds  in  some  copper  ores. 
Its  origin  is  secondary  in  all  cases.  It  occurs  in  the  Tintic  district, 
Utah,  and  in  many  copper  veins  in  Europe  and  in  South  America. 

Libethenite  occurs  in  compact  or  globular  masses  and  in  small 
crystals  that  resemble  those  of  olivenite.    Their  axial  ratio  is  .9605  : 
1  :  .7019  and  no  A  110=87°  40'. 

The  mineral  is  brittle.  Its  fracture  is  indistinctly  conchoidal.  Its 
color  is  dark  olive-green  and  its  streak  a  lighter  shade.  It  is  translucent 
or  transparent  and  has  a  resinous  luster.  Its  hardness =4  and  sp.  gr. 
=  3.7.    Its  intermediate  refractive  index  for  yellow  light  is  1.743. 

When  heated  in  the  closed  tube  it  yields  water  and  blackens.  It  is 
easily  fusible  (2).  It  yields  the  usual  reaction  for  Cu  and  P,  and  is  sol- 
uble in  acids  and  in  ammonia.  It  is  distinguished  from  olivenite  by  the 
reaction  for  phosphorus. 

It  occurs  at  many  of  the  localities  for  olivenite,  where,  like  this  min- 
eral, it  is  a  decomposition  product  of  other  copper  compounds. 


Herderite  (CaBe(OH  F)P04) 

Herderite  is  an  isomorphous  mixture  of  the  two  phosphates,  CaBeFP04 
and  CaBe(OH)P04.  The  latter  molecule  occurs  in  nature  as  hydro- 
herderite\  the  former  occurs  only  in  mixtures.  The  theoretical  compo- 
sition of  the  fluorine  (I)  and  hydroxyl  (II)  molecules  and  of  transparent 
crystals  from  Stoneham  (III),  and  Paris  (IV),  Maine,  are  given  below: 


BeO 

CuO 

P2O5 

F 

H20 

Ins. 

Total 
(lessO=F) 

I. 

15-39 

3433 

4353 

11.64 

.  *  •   . 

•   •   • 

100 

II. 

IS- 53 

34.78 

44.10 

•   •  •   • 

5-59 

•        B        • 

100 

III. 

i5-5i 

3367 

4374 

S-27 

3-7° 

■         •        • 

99.67 

IV. 

16.13 

34  04 

44  05 

5-85 

•      •      •      • 

■44 

100.51 

The  mineral  is  found  only  in  crystals,  which  are  monoclinic,  with 
alb:  £=.6301  :  1  :  .4274  and  18=89°  54'.  Their  habit  is  hexagonal, 
pyramidal  or  short  prismatic,  elongated  in  the  direction  of  a. 


PHOSPHATES,  ARSENATES  AND  VANADATES     279 

Herderite  is  colorless  or  light  yellow,  transparent  or  translucent. 
Its  refractive  indices  are:  a=  1.592,  0=  1.612,  7=  1.621. 

Its  density  is  about  3,  diminishing,  as  the  amount  of  hydroxyl  in- 
creases, to  2.952  in  the  pure  hydroherderite. 

Before  the  blowpipe  herderite  first  phosphoresces  with  an  orange- 
yellow  light,  then  fuses  to  a  white  enamel,  colors  the  flame  red  and  yields 
fluorine.  In  the  closed  glass  tube  most  specimens  yield  an  acid  water, 
which,  when  strongly  heated,  evolves  fluorine  that  etches  the  glass. 
The  mineral  also  reacts  for  phosphorus  with  magnesium  ribbon.  It  is 
slowly  soluble  in  HC1. 

Occurrence,  Origin  and  Uses. — Herderite  occurs  in  pegmatite  dikes 
at  Stoneham,  Hebron,  and  other  places  in  Maine,  and  at  the  tin  mines  of 
Ehrenfriedersdorf,  Saxony;  in  all  of  these  places  it  is  apparently  of 
pneumatolytic  origin.  The  material  from  Maine  is  used  to  a  small 
extent  as  a  gem  stone. 

ACID  PHOSPHATES 

Acid  phosphates  are  those  in  which  all  of  the  hydrogen  atoms  of  the 
acids  have  not  been  replaced  by  metals  or  by  basic  radicals.  Theoret- 
ically, they  contain  replaceable  hydrogen  atoms.  There  are  12  or  15 
minerals  that  are  thought  to  belong  to  this  class,  but  the  composition 
of  many  of  them  is  very  obscure.  Most  of  them  appear  to  be  hydrated. 
The  only  important  mineral  that  may  belong  to  the  class  is  the  popular 
gem  stone,  turquoise.  This,  according  to  the  best  analyses,  contains  its 
components  in  the  proportions  indicated  by  the  formula  CuO,  3AI2O3, 
2P2O5, 9H2O,  which  may  be  interpreted  as  (CuOH)(Al(OH)2)6H5(P04)4. 
which  is  4(HaP04),  in  which  6  hydrogen  atoms  are  replaced  by  6A1(0H)2 
groups  and  one  by  the  group  CuOH. 

Turquoise  ((CuOH)(Al(OH)2)6H5(P04)4) 

Turquoise  is  apparently  a  definite  compound  of  the  formula  indicated 
above,  which  requires  34.12  per  cent  P2O5,  36.84  per  cent  AI2O3,  9.57 
per  cent  CuO  and  19.47  per  cent  H2O.  Analysis  of  a  crystallized  variety 
from  Lynch,  Campbell  Co.,  Virginia,  gave: 


P205 

A1203 

Fe203 

CuO 

H20 

Total 

* 

34.13 

36.50 

.21 

9.00 

20.12 

99.96 

Most  specimens,  however,  have  not  as  simple  a  composition  as  this. 
They  are  probably  isomorphous  mixtures  of  unidentified  phosphates. 


280  DESCRIPTIVE  MINERALOGY 

The  mineral  as  usually  found  is  apparently  an  amorphous  or  cryp- 
tocrystalline,  translucent  or  opaque  material  with  a  waxy  luster  and  a 
sky-blue,  green  or  greenish  gray  color.  Material  recently  found  at 
Lynch,  Virginia,  however,  occurs  in  minute  triclinic  crystals  with  an 
axial  ratio  .7910  :  1  :  .6051,  with  0=87°  02',  0=86°  29',  and  7=72°  19'- 
Their  habit  is  pyramidal  with  00 Poo  (100),  00 P 60  (010),  oo'P(iTo), 
ooP'(no)  and  Poo  (0T1). 

The  fracture  of  turquoise  is  conchoidal.  It  has  a  hardness  of  5-6 
and  a  specific  gravity  between  2.61  and  2.89.  It  is  brittle,  and  has  cleav- 
ages in  two  directions.  The  determined  refractive  indices  of  the  Vir- 
ginia crystals  are:  a=  1.61,  7=  1.65. 

In  the  closed  tube  the  mineral  decrepitates,  yields  water  and  turns 
black  or  brown.  It  is  infusible,  but  it  assumes  a  glassy  appearance  when 
heated  before  the  blowpipe  and  colors  the  flame  green.  When  moistened 
with  HC1  and  again  heated  the  flame  is  tinged  with  the  azure  blue  of 
copper  chloride.  The  mineral  reacts  for  copper  and  phosphoric  acid. 
Some  specimens  dissolve  in  HC1,  but  the  crystallized  material  from  Vir- 
ginia is  insoluble  until  after  it  is  strongly  ignited.  It  partly  dissolves 
in  KOH,  with  the  production  of  a  brown  residue  of  a  copper  compound. 

Occurrence. — Turquoise  occurs  in  thin  veins  cutting  through  certain 
decomposed  volcanic  rocks  and  other  rocks  in  contact  with  them, 
and  in  grains  disseminated  through  them,  in  stalactites,  globular 
masses  and  crusts.  It  is  probably  an  alteration  product  of  other  com- 
pounds. 

Localities. — Turquoise  is  found  in  narrow  veins  and  irregular  masses 
in  the  brecciated  portions  of  acid  volcanic  rocks  and  the  surrounding  clay 
slates,  near  Nishapur,  in  Persia;  in  the  Megara  Valley,  Sinai,  and  near 
Samarkand,  in  Turkestan.  In  all  these  places  the  mineral  is  of  gem 
quality  and  until  recently  nearly  all  the  gem  turquoise  came  from  them. 
Within  late  years  gem  turquoise  has  been  discovered  in  the  Cerillo  Moun- 
tains, near  Santa  F6,  New  Mexico,  where  it  has  been  mined  in  consid- 
erable quantity.  The  locality  is  the  site  of  an  ancient  mine  which  was 
worked  by  the  Mexicans.  It  is  also  found  and  mined  in  the  Burro 
Mountains,  Grant  County,  in  the  same  State,  near  Millers,  and  at  other 
points  in  Nevada  and  near  Mineral  Park,  Mohave  County,  Arizona, 
where  also  the  ancient  Mexicans  once  had  mines.  At  La  Jara,  Conejos 
County,  Colorado,  old  mines  have  likewise  been  opened  up  and  are  now 
yielding  gem  material. 

Uses. — The  only  use  of  turquoise  is  as  a  gem  stone.  Though  much 
of  the  American  mineral  is  pale  or  green,  some  of  it  is  of  as  fine  color  as 
the  Oriental  stone.     A  favorite  method  of  using  the  stone  is  in  its 


PHOSPHATES,  ARSENATES  AND  VANADATES     281 

matrix.     Small  pieces  of  the  rock  with  its  included  turquoise  are  pol- 
ished and  sold  under  the  name  of  turquoise  matrix. 

Production. — The  total  value  of  the  turquoise  and  turquoise  matrix 
produced  in  the  United  States  during  191 1  was  $44,751.  This  weighed 
about  4,363  pounds.  In  several  previous  years  the  production  reached 
about  $150,000,  but  in  1912  it  was  valued  at  only  $10,140. 

HYDROUS  PHOSPHATES  AND  ARSENATES 

HYDRATED  NORMAL  PHOSPHATES  AND  ARSENATES 

Of  the  hydrous  salts  of  orthophosphoric  and  orthoarsenic  acids  there 
are  two  which  are  of  some  importance  because  they  are  fairly  common, 
a  third  which  is  utilized  in  jewelry,  and  a  fourth  that  is  important  as  an 
indicator  of  the  presence  of  an  ore  of  cobalt.  The  first  two  are  vivianite 
and  scorodite,  a  phosphate  and  an  arsenate  of  iron,  the  third  is  variszite, 
an  aluminium  phosphate,  and  the  fourth  is  erythrite,  an  arsenate  of 
cobalt.  A  dimorph  of  variscite,  known  as  lucinite,  is  rare.  All  give 
water  in  the  closed  tube  and  yield  phosphine  when  fused  with  magne- 
sium and  moistened  with  water. 

VIVIANITE  GROUP 

The  only  important  group  of  the  hydrated  orthophosphates  and 
orthoarsenates  is  that  of  which  vivianite  and  erythrite  are  members. 
The  general  formula  of  the  group  is  R"3(R'"04)2-8H20  in  which  R" 
=  Fe,  Co,  Ni,  Zn  and  Mg,  and  R"'=P  or  As.  Although  some  members 
have  not  been  found  in  measurable  crystals,  crystals  of  all  have  been 
made  in  the  laboratory,  so  that  there  is  little  doubt  of  their  isomorphism. 
All  are  monoclinic  prismatic  with  axial  ratios  of  about  .75  :  1  :  .70  and 
P  about  740.    The  group  is  as  follows: 

Bobierite,  Mg3 (P04)2  •  8H20  Erythrite,  C03 (As04)2  •  8H20 

Hornesite,  Mg3 (As04)2  ■  8H2O  A nnabergite,  Nia (As04)2  •  8H20 

Vivianite,  Fe3 (P04)2  •  8H20  Cabrerite,  (Ni  •  Mg)3  ( As04)2  •  8H20 

Symplesite,  Fe3(As04)2 •  8H20  Kottigite,  Zn3(As04)2 •  8H20 

Only  vivianite,  erythrite  and  annabergite  are  described. 

Vivianite  (Fe3(P04)2-8H20) 

Vivianite  is  a  common  phosphate  of  iron.  It  occurs  not  only  in  dis- 
tinct crystals  but  also  as  bluish  green  stains  on  other  minerals,  and  as 
an  invisible  constituent  of  certain  iron  ores,  thereby  diminishing  their 
value. 


282  DESCRIPTIVE  MINERALOGY 

Its  formula  indicates  the  presence  of  43  per  cent  FeO,  28.3  per  cent 
P2O5  and  28.7  per  cent  H2O. 

Vivianite  crystals  are  monoclinic  (prismatic  class),  usually  with  a 
prismatic  habit.  Their  axial  ratio  is  .7498  :  1  :  .7015,  and  18=75°  34'. 
The  principal  forms  observed  on  them  are  00  P  56  (100),  00  P  00  (010), 
oop(no),  °oP3(3io),  P*(ioi),  P(in)  and  oP(ooi).  The  angle 
110A  110=71°  58'.  The  mineral  also  occurs  in  stellate  groups,  in  glob- 
ular, fibrous  and  earthy  masses  and  as  crusts  coating  other  compounds. 

Its  cleavage  is  perfect  parallel  to  00 Pod  (010).  It  is  flexible  in 
thin  splinters  and  sectile.  The  fresh,  pure  mineral  is  colorless  and  trans- 
parent, but  specimens  usually  seen  are  more  or  less  oxidized  and  have 
a  blue  or  green  color.  It  has  a  vitreous  to  pearly  luster.  Its  streak  is 
white  or  bluish,  changing  to  indigo-blue  or  brown  on  exposure  to  the  air. 
Its  pleochroism  is  strong  in  blue  and  pale  yellow  tints.  Its  hardness 
is  1.5-2  and  density  about  2.6.  Its  refractive  indices  for  yellow  light 
are:  a=  1. 5818,  0=1.6012,  7=1.6360. 

In  the  closed  tube  vivianite  whitens,  exfoliates  and  yields  water  at  a 
low  temperature.  It  fuses  easily  (2),  tingeing  the  flame  bluish  green. 
Its  fusion  temperature  is  n  14°.  The  fused  mass  forms  a  grayish  black 
magnetic  globule.    It  gives  the  reaction  for  iron,  and  is  soluble  in  HC1. 

The  mineral  is  easily  recognized  by  its  softness,  easy  fusibility  and 
by  yielding  the  test  for  phosphorus. 

Synthesis. — Crystals  have  been  made  by  heating  iron  phosphate  with 
a  great  excess  of  sodium  phosphate  for  eight  days. 

Occurrence  and  Origin. — Vivianite  occurs  in  veins  of  copper,  tin  and 
gold  ores;  disseminated  through  peat,  clay,  and  limonite;  coating  the 
walls  of  clefts  in  feldspars  and  other  minerals  of  certain  igneous  rocks, 
and  partially  filling  cavities  in  fossils  and  partly  fossilized  bones.  It  is 
usually  the  result  of  the  decomposition  of  other  minerals. 

Localities. — Crystals  are  found  at  several  points  in  Cornwall,  Eng- 
land; at  the  gold  mines  at  Verespatak,  in  Transylvania;  at  Allentown, 
Monmouth  County,  New  Jersey,  and  at  many  other  places.  The  earthy 
variety  occurs  at  Allentown,  Mullica  Hill  and  other  points  in  New  Jer- 
sey, in  Stafford  County,  Virginia,  and  in  swamp  deposits  at  many  places. 
It  is  abundant  in  limonite  at  Vaudreuil,  in  Quebec,  and  in  bog  iron  ores 
elsewhere. 

Erythrite  (Co3(As04)2-8H20) 

Erythrite,  or  cobalt  bloom,  is  not  a  common  mineral,  but,  because 
of  its  beauty  and  the  fact  that  it  is  the  usual  alteration  product  of  cobalt 
ores,  it  deserves  to  be  described. 


PHOSPHATES,   ARSENATES  AND  VANADATES      283 

In  composition  erythrite  is  37.5  per  cent  CoO,  38.4  per  cent  AS2O5, 
and  24.1  per  cent  H2O.  It  usually,  however,  contains  some  iron,  nickel 
and  calcium. 

The  mineral  is  isomorphous  with  vivianite.  Its  crystals  are  mono- 
clinic  and  prismatic  or  acicular  and  their  axial  ratio  is  .7937  :  1  :  .7356 
and  0=74°  51'.  The  prisms  are  striated  vertically.  Erythrite  occurs 
in  all  the  forms  in  which  vivianite  is  found.  Its  crystals  are  usually 
bounded  by    00 P 00  (010),    ooP(no),    00 Poo  (100),   +P<x>(Toi)  and 

P(Tn). 

The  cleavage  of  erythrite  is  perfect  parallel  to  00  P  00  (010).    It  is 

transparent  or  translucent,  has  a  gray,  crimson  or  peach-red  color, 

and  a  white  or  pink  streak.     Its  hardness  varies  between  1.5  and  2.5 

and  its  density  is   2.95.     Its   luster   is   pearly  on  00  P  00  (010)   and 

vitreous  on  other  faces.     It   is  flexible  and  sectile.     Its  refractive 

indices  for  yellow  light  are:  a=  1.6263,  0=  1.6614,  7=  1.6986. 

In  the  closed  tube  erythrite  turns  blue  and  yields  water  at  a  low  tem- 
perature. At  a  high  temperature  it  yields  AS2O3,  which  condenses  in 
the  cold  portion  of  the  tube  as  a  dark  sublimate.  It  fuses  at  2,  and 
tinges  the  flame  pale  blue.  On  charcoal  it  fuses,  yields  arsenic  fumes  and 
a  gray  globule  which  colors  the  borax  bead  a  deep  blue.  The  mineral 
is  soluble  in  HC1,  giving  rise  to  a  pink  solution,  which,  upon  evaporation 
to  dryness,  gives  a  blue  stain. 

It  is  easily  recognized  by  its  color  and  the  cobalt  reaction.  It  is 
readily  distinguished  from  pink  tourma.ine  (p.  434),  by  its  hardness 
and  easy  fusibility. 

Synthesis. — Crystals  have  been  obtained  by  carefully  mixing  to- 
gether warm  solutions  of  C0SO4  and  HNa2As04  •  7H2O. 

Occurrence. — Erythrite  occurs  in  the  upper  portions  of  veins  con- 
taining cobalt  minerals,  being  formed  by  their  weathering. 

Localities. — It  occurs  as  scales  and  crystals  at  Schneeberg,  Saxony, 
and  as  crystals  at  Modum,  Norway.  It  is  found,  also,  at  Lovelock's 
Station,  Nevada,  at  several  points  in  California  and  in  large  quantities 
at  Cobalt,  Ontario. 

Annabergite  (Ni3(As04)2-8H20) 

Annabergite,  or  nickel  bloom,  is  isomorphous  with  erythrite.  It 
occurs  massive,  disseminated  in  tiny  grains  through  certain  rocks,  as 
crusts  and  stains  in  globular  and  earthy  masses,  and  in  fibrous  crystals, 
the  axial  ratios  of  which  are  not  known. 

The  mineral  is  apple-green  in  color,  and  is  translucent  or  opaque. 


284  DESCRIPTIVE  MINERALOGY 

Its  streak  is  light  green.  Its  luster  is  vitreous;  its  hardness,  1.5-2.5 
and  sp.  gr.  =  3. 

Before  the  blowpipe  it  melts  to  a  gray  globule  and  gives  the  arsenic 
odor.  In  the  closed  glass  tube  it  blackens  and  yields  water.  In  the 
beads  it  gives  the  usual  reactions  for  Ni.  The  mineral  dissolves  easily 
in  acids. 

Synthesis. — Crystals  have  been  produced  by  the  method  employed 
in  the  synthesis  of  erythrite,  using  NiSQt,  instead  of  C0SO4. 

Occurrence. — It  is  found  as  a  common  alteration  product  of  nickel- 
bearing  minerals,  in  the  oxidized  portions  of  veins. 

Localitus. — Its  best  known  occurrences  are  in  Allemont,  Dauphin^; 
Annaberg  and  Schneeberg,  Saxony;  Cobalt,  Ontario,  and  mines  in 
Colorado  and  Nevada. 


Variscite  (AIPO42H2O) 

Variscite  is  a  bright  green  mineral  that  has  recently  come  into  use  as 
a  gem  material.  It  is  apparently  an  aluminium  phosphate  with  a 
theoretical  composition  as  follows:  44.9  per  cent  P2O5;  32.3  per  cent 
AI2O3  and  22.8  per  cent  H2O.  A  specimen  of  crystallized  material  from 
Lucin,  Utah,  gave  the  following  analysis: 


P205 

A1203 

Fe203 

Cr03 

V2O3 

H20 

Total 

44.73 

32-40 

.06 

.18 

•32 

22.68 

100.37 

Recent  investigations  indicate  that  the  compound  A1P04-2H20  is 
dimorphous.  Both  forms  are  orthorhombic  but  one,  variscite,  has  the 
properties  described  under  this  heading.  The  other,  lucinite,  is  associ- 
ated with  variscite,  near  Lucin,  Utah.  It,  however,  occurs  in  crystals 
that  are  octahedral  in  habit,  rather  than  tabular,  and  that  have  an 
axial  ratio  of  .8729  :  1  :  .9788.  In  other  respects  lucinite  is  very  much 
like  variscite. 

An  amorphous  variety  of  the  same  substance  is  also  kaiown.  It 
occurs  as  a  white,  pale  brown  or  pale  blue  earthy  mass  with  a  sp.  gr.  of 
2.135.  I*  differs  from  the  crystalline  varieties  in  being  completely 
soluble  in  warm  concentrated  H2SO4. 

The  crystals  of  variscite  are  orthorhombic  and  are  bounded  by 
00  P  60  (010),  00  P(no)  and  ^P  06  (012),  and  in  a  few  cases  00  P  60  (100). 
Their  axial  ratio  is  .8944  :  1  :  1.0919.  Nearly  all  crystals  are  tabular 
parallel  to  00  P  60  (010).  Twins  are  common,  with  ^P  60  (102)  the 
twinning  plane.  »  Crystals  are  comparatively  rare,  the  mineral  occur- 
ring usually  in  fibrous  or  finely  granular  masses  and  as  incrustations. 


PHOSPHATES,   ARSENATES  AND  VANADATES     285 

Variscite  varies  in  color  from  a  pale  to  a  bright  green.  It  is  weakly 
pleochroic,  has  a  vitreous  luster,  a  hardness  of  about  4  and  a  density  of 
2.54.    Its  refractive  indices  for  yellow  light  are:  a=  1.546,  18=1.556, 

y=  1-578- 

Before  the  blowpipe  the  mineral  is  infusible.  It,  however,  whitens 
and  colors  the  flame  deep  bluish  green.  It  yields  water  in  the  closed 
tube,  and  with  the  loss  of  its  water,  it  changes  color  from  green  to 
lavender.  The  same  change  in  color  takes  place  gradually  at  temper- 
atures between  iio°-i6o°.  When  heated  with  Co(N03)2,  it  turns  blue 
and  when  fused  with  magnesium  ribbon  it  gives  the  test  for  phosphorus. 
It  forms  a  yellowish  green  glass  with  borax  or  microcosmic  salt.  The 
mineral  is  insoluble  in  acids  before  heating. 

Variscite  resembles  in  some  respects  certain  varieties  of  turquoise 
and  wavelliie  (p.  287).  It  is  distinguished  from  turquoise  by  the  absence 
of  copper  and  from  wavellite  by  its  insolubility  in  acids. 

Occurrence. — The  mineral  occurs  as  a  cement  in  a  brecciated,  cherty 
limestone  and  a  brecciated  rhyolite,  as  nodules  in  the  cherty  portions 
of  the  breccias  and  also  as  veins  traversing  these  rocks.  It  is  also 
found  as  nests  in  weathered  pegmatites.  The  crystals  occur  as  coarsely 
granular,  loosely  coherent  masses  in  more  compact  granular  masses. 

Localities. — Variscite  occurs  at  Messbach,  Saxony;  in  Montgomery 
County,  Arkansas;  near  Lucin,  Utah,  and  at  a  number  of  other  places 
in  Tooele  and  Washington  Counties  in  this  State;  in  Esmeralda  County, 
Nevada,  and  in  Montgomery  County,  Arkansas.  The  colloidal  variety 
occurs  as  concretions  in  slates  at  Brandberg,  near  Leoben,  Austria. 

Uses. — The  mixture  of  variscite  and  rock  is  cut,  and  employed  as 
sets  in  necklaces,  belt  pins,  etc.,  under  the  names  "  utahlite  "  and 
"  amatrice,"  but  because  of  the  softness  of  the  variscite  it  cannot  be 
used  with  success  for  all  the  purposes  for  which  turquoise  matrix  is 
used. 

Production. — The  production  of  the  material  in  the  United  States 
during  191 1  was  540  lb.,  valued  at  $5,750.  In  the  previous  year 
5,377  lb.  were  reported  as  having  been  sold  for  $26,125.  In  191 2, 
the  amount  marketed  was  valued  at  $8,150. 

Skorodite  (FeAs(V  2H2O) 

Skorodite  is  more  common  than  vivianite.  It  occurs  in  globular 
and  earthy  masses,  as  incrustations,  and  in  crystals  of  a  green  or  brown 
color.    The  globular  forms  are  colloidal. 

Its  formula  indicates  Fe2C>3  =  34.6  per  cent,   As203  =  49-8  per  cent 


286 


DESCRIPTIVE  MINERALOGY 


and  HaO=  15.6  per  cent.    An  incrustation  on  the  deposits  of  the  Joseph's 
Coat  Spring,  Yellowstone  National  Park,  consisted  of: 


As205 

Fe203 

H20 

Si02 

SO3 

Total 

46.48 

33  29 

15.50 

435 

.84 

100.46 

Its  crystallization  is  orthorhombic  (bipyramidal  class),  with  a  :  b  :  c 
=  .8658  :  1  :  .9541.    The  crystals,  which  are  commonly  bounded  by 

00  P  56  (lOo),     00  P06  (oio),     00P2(l2o),      OOP(HO), 

P(iii)  and  £P(ii2),  are  either  prismatic  or  octa- 
hedral in  habit   (Fig.  159).    The  angle  111A1I1 

=  65°  20'.     Their  cleavage  is  imperfect,  parallel  to 

ooP(no). 

The  mineral  is  brittle.  It  has  a  vitreous  luster, 
a  leek-green  or  liver-brown  color  and  a  white 
streak.  It  is  translucent  and  has  an  uneven  frac- 
ture.    Its  hardness  is  3.5-4  and  density  about  3.3. 

Fig.  150.  — Skorodite  ^he  colloidal  phases  are  somewhat  softer  than  the 
Crystal  with  00  P  00  ,  .   ...         , 

,  .        ~-  crystalline  phases. 

100  (a);    00  P2,  120      J  11 

(d),  and  P,  in  (p)         *n  tne  c*osed  tube  skorodite  turns  yellow  and 

yields  water.  It  fuses  easily,  coloring  the  flame 
bluish.  On  charcoal  it  yields  white  arsenical  fumes  and  gives  a  black 
porous,  magnetic  button.    It  is  soluble  in  HC1,  forming  a  brown  solution. 

It  is  distinguished  from  livianite  by  the  arsenic  test,  and  from  dufren- 
ite  by  its  streak  and  reaction  in  the  closed  tube. 

Synthesis. — Skorodite  crystals  have  been  made  by  heating  metallic 
iron  with  concentrated  arsenic  acid  solution  at  i40°-i5o°. 

Occurrence. — Skorodite  is  frequently  associated  with  arsenopyrite, 
in  the  oxidized  portions  of  veins  containing  iron  minerals.  It  is  found 
also  in  a  few  places  as  incrustations  deposited  by  hot  springs. 

Localities. — It  occurs  in  fine  crystals  at  Nerchinsk,  Siberia;  at 
Loelling,  in  Carinthia;  near  Edenville,  New  York;  in  the  Tintic  dis- 
trict, Utah,  and  as  an  incrustation  on  the  siliceous  sinter  of  the  geysers 
in  Yellowstone  Park. 


HYDRATED  BASIC  PHOSPHATES  AND  ARSENATES 

The  hydrated  basic  phosphates  and  arsenates  are  rather  more  nu- 
merous than  the  hydrated  normal  compounds,  but  most  of  them  are  rare. 
One,  wavellite,  however,  is  a  handsome  mineral  that  is  fairly  common. 
Another,  pkarmacosiderite,  an  iron  arsenate,  is  known  to  occur  at  a 
number  of  places.     The  uranite  group  also  belongs  here.     Its  members 


PHOSPHATES,  ARSENATES  AND  VANADATES     287 

are  comparatively  rare,  but,  because  of  the  presence  of  uranium  in  them, 
they  are  of  considerable  interest. 

WaveUite  ((Al(OH-F)3)(P04)a-5HsO) 
Wavellite  rarely  occurs  in  crystals.    It  is  usually  in  acicular  aggre- 
gates that  are  either  globular  or  radiating  (Fig.  160).    The  few  crystals 
that  have  been  seen  are  orthorhombic  (bipyramidal  class),  with  an 
axial  ratio  of  .5573  :  1  :  .4057. 

Its  composition  varies  widely,  and  frequently  a  fairly  large  portion 
of  the  OH  is  replaced  by  F,  and  a  portion  of  the  Al  by  Fe. 

The  mineral  is  vitreous  in  luster  and  white,  green,  yellow,  brown  or 
black  in  color.    Its  streak  is  white.    It  is  brittle  and  translucent,  in- 


Fig.  160.— Radiate  Wavellile  on  a  Rock  Surface. 

fusible  and  insoluble  in  acids.  Its  hardness  is  3.5  and  its  density  3.41. 
Its  intermediate  refractive  index  for  yellow  light  is  1.526. 

Heated  in  a  dosed  glass  tube,  wavellite  yields  water,  the  last  traces 
of  which  react  acid  and  often  etch  the  glass.  In  the  blowpipe  flame  the 
mineral  swells  up  and  breaks  into  tiny  infusible  fragments,  at  the  same 
time  tingeing  the  flame  green.  The  mineral  is  soluble  in  HC1  and 
H2SO4.  When  heated  with  H2SO4  many  specimens  yield  hydrofluoric 
acid.  When  heated  on  charcoal  and  moistened  with  Co(N03)2  and 
reheated,  the  mineral  turns  blue. 

Wavellite  is  distinguished  from  turquoise,  which  it  sometimes 
resembles,  by  its  action  in  the  blowpipe  flame,  by  its  inferior  hardness 
and  its  manner  of  occurrence. 

Occurrence- — Wavellite  occurs  as  radiating  bundles  on  the  walls  of 


288 


DESCRIPTIVE  MINERALOGY 


cracks  in  various  rocks  and  as  globular  masses  filling  ore  veins  and  the 
spaces  between  the  fragments  of  breccias.  It  is  probably  in  all  cases 
the  result  of  weathering. 

Localities. — It  is  found  at  a  great  number  of  places,  especially  at 
Zbirow,  in  Bohemia;  at  Minas  Geraes,  Brazil;  at  Magnet  Cove,  Arkan- 
sas, and  in  the  slate  quarries  in  York  County,  Penn. 

Pharmacosiderite  ((FeOH)3(As04)2  5H2O) 

Pharmacosiderite  is  a  hydrated  ferric  arsenate,  the  composition  of 
which  is  not  firmly  established.  It  usually  occurs  in  small  isometric 
crystals   (hextetrahedral  class),   that  are  commonly  combinations  of 

00 O 00  (100)   and  —(in).    It  is  also   sometimes  found  in  granular 

2 

masses.    Its  cleavage  is  parallel  to  00  O  00  (100). 

The  mineral  is  green,  dark  brown  or  yellow.  Its  streak  is  a  pale 
shade  of  the  same  color.  It  has  an  adamantine  luster  and  is  translucent. 
Its  hardness  =  2.5  and  sp.  gr.  =  3.  It  is  sectile  and  pyroelectric.  Its 
refractive  index,  n=  1.676. 

Pharmacosiderite  reacts  like  skorodite  before  the  blowpipe  and  with 
reagents. 

The  mineral  occurs  in  the  oxidized  portions  of  ore  veins,  in  Cornwall, 
England;  at  Schneeberg,  Saxony;  near  Schemnitz,  Hungary;  and  in  the 
Tintic  district,  Utah. 

URANITE  GROUP 

The  uranites  are  a  group  of  phosphates,  arsenates  and  vanadates 
containing  uranium  in  the  form  of  the  radical  uranyl  (UO2)  which  is 
bivalent.  The  members  of  the  group  are  either  tetragonal,  or  ortho- 
rhombic  with  a  tetragonal  habit.  They  all  contain  eight  molecules  of 
water  of  crystallization.  Only  three  members  of  the  group  are  of 
sufficient  interest  to  be  discussed  here.  These  are  the  hydrated  cop- 
per and  calcium  uranyl  phosphates,  torbernite  and  autunite  and  the 
potassium  uranyl  vanadate,  carnotite. 

The  entire  group  so  far  as  its  members  have  been  identified  is  as 
follows: 


Autunite 

Uranospinite 

Torbernite 

Zeunerite 

Uranocircite 

Carnotite 


Ca(U02)2(P04)2-8H20 

Ca(U02)2(As04)2-8H20 

Cu(U02)2(P04)2-8H20 

Cu(U02)2(As04)2-8H20 

Ba(U02)2(P04)2-8H20 

(Ca  •  K2)  (U02)2(V04)2  •  xHaO 


Orthorhombic 

Orthorhombic 

Tetragonal 

Tetragonal 

Orthorhombic 


k 


PHOSPHATES,  ARSENATES  AND  VANADATES     289 

The  uranites  are  of  interest  because  of  their  content  of  uranium,  an 
element  which  is  genetically  related  to  radium. 

Autunite  (Ca(U02)2(P04)2-8H20) 

Autunite  occurs  in  thin  tabular  crystals  with  a  distinctly  tetragonal 
habit,  and  in  foliated  and  micaceous  masses. 

The  percentage  composition  corresponding  to  the  above  formula 
is  6.i  per  cent  CaO,  62.7  per  cent  UO3,  15.5  per  cent  P2O5  and  15.7  per 
cent  H2O. 

Its  crystals  are  orthorhombic  (bipyramidal  class),  with  an  axial 
ratio,  .9875  :  1  :  2.8517,  thus  possessing  interfacial  angles  that  closely 
approach  those  of  torbernite.  Its  crystals  are  bounded  by  oP(ooi), 
Poo  (101),  P06  (on),  and  several  less  prominent  planes.  Their  cleav- 
age is  very  perfect  and  the  cleavage  lamellae  are  brittle.  The  luster  is 
pearly  on  the  base  and  vitreous  on  other  surfaces. 

The  mineral  is  lemon-yellow  or  sulphur-yellow  in  color,  and  its  streak 
is  yellow.  It  is  transparent  to  translucent.  Its  hardness  is  2-2.5  and 
its  specific  gravity  about  3.2.    Its  refractive  indices  for  yellow  light  are: 

«  =  i.553>  0=i-575>  7=1.577- 

The  mineral  reacts  like  torbernite  before  the  blowpipe  and  with  acids, 

except  that  it  shows  none  of  the  tests  for  copper.     It  is  recognized  by  its 

color,  streak  and  specific  gravity. 

Occurrence. — Autunite  occurs  in  pegmatite  veins  and  on  the  walls 
of  cracks  in  rocks  near  igneous  intrusions,  especially  in  association  with 
other  uranium  compounds,  of  which  it  is  a  decomposition  product. 

Localities. — It  has  been  found  at  Johanngeorgenstadt,  Germany; 
at  Middletown  and  Branch ville,  Conn.,  in  the  mica  mines  of  Mitchell 
County,  North  Carolina,  and  coating  cracks  in  gneiss  at  Baltimore,  Md. 

Torbernite  (Cu(U02)2(P04)2  -8H20) 

Torbernite  occurs  in  small  square  tables,  that  may  be  very  thin  or 
moderately  thick,  and  in  foliated  and  micaceous  masses. 

The  pure  mineral  contains  61.2  per  cent  UO3,  8.4  per  cent  Cu, 
15.1  per  cent  P2O5  and  15.3  per  cent  H2O,  but  frequently  a  part  of  the  P 
is  replaced  by  As. 

Its  crystals  are  tetragonal  (ditetragonal  bipyramidal  class),  with 
a  :  c=i  :  2.9361.  They  are  extremely  simple,  their  predominating 
forms  being  oP(ooi)  and  Poo  (101).  Less  prominent  are  00  Poo  (100), 
2Poo(2oi)  and  ooP(no).  Their  cleavage  is  perfect  parallel  to  oP. 
The  cleavage  lamellae  may  be  almost  as  thin  as  those  of  the  micas 
but  they  are  brittle. 


290  DESCRIPTIVE  MINERALOGY 

The  mineral  is  bright  green  in  emerald,  grass  or  apple  shades,  has  a 
lighter  green  streak,  is  translucent  or  transparent,  and  has  a  hardness 
of  2.25  and  a  specific  gravity  of  about  3.5.  Its  luster  is  pearly  on  the 
basal  plane,  but  nearly  vitreous  on  other  surfaces.  It  is  strongly  pleo- 
chroic  in  green  and  blue. 

Torbernite  gives  reactions  for  Cu  and  P  and  yields  water  in  the 
closed  tube.  The  bead  reactions  for  uranium  are  masked  by  those  of 
copper.    The  mineral  is  soluble  in  HNO3. 

The  mineral  is  easily  recognized  by  its  color  and  other  physical 
properties. 

Occurrence. — Torbernite  is  occasionally  found  as  a  coating  on  the 
walls  of  crevices  in  rocks.  It  occurs  in  Cornwall,  England;  at  Schee- 
berg,  Saxony;  at  Joachimsthal,  Bohemia,  and  at  most  places  where  other 
uranium  minerals  exist.    It  is  probably  in  all  cases  a  weathering  product. 

Carnotite  ((Ca-K2)(U02)2(V04)2-xH20) 

Carnotite,  like  the  other  uranites  described,  is  extremely  complex 
in  composition.  It  may  be  an  impure  potassium  uranyl  vanadate,  or  a 
mixture  of  several  vanadates  in  which  the  potassium  uranyl  compound 
is  the  most  prominent.  The  formula  given  above  indicates  its  com- 
position as  well  as  any  simple  formula  that  has  been  proposed.  A 
specimen  from  La  Sal  Creek,  Colorado,  shows  the  mineral  to  be  essen- 
tially as  follows: 

« 

V2O5       U03        CaO      BaO      K20      H20  at  105  °  H20  above  105  ° 
18.05      54 -00      I-^°      J-86      5  46  3.16  2.21 

though  there  are  present  in  the  specimen  analyzed,  or  in  other  specimens 
from  the  same  locality,  also  As203,  P2Os,  Si02,  Ti02,  C02,  SO3,  M0O3, 
Cr203,  Fe203,  A1203,  PbO,  CuO,  SrO,  MgO,  Li20  and  Na20,  and  there 
are  reported  in  them  also  small  quantities  of  radium.  Radiographs 
taken  with  the  aid  of  carnotite  have  been  published,  which  are  almost 
as  clear  as  those  taken  with  pitchblende.  The  complete  analysis  of  a 
specimen  from  the  Copper  Prince  Claim,  Montrose  Co.,  Colo.,  gave: 


v2o5 

As205 

P205 

UO3 

M0O3 

Fe203 

Al20s    PbO 

18.35 

25 

•33 

5225 

•23 

1.77 

1.08       .25 

CuO 

CaO 

BaO 

K20 

H20- 

H20+ 

Ins.         Total 

.20 

2.85 

.72 

6-73 

2-59 

3.06 

8.34        99.84 

Also  Ti02  =  .io;    C02  =  33;    S03  =  .i2;    Cr03  =  tr.;    MgO=.2o  and 
Na20=.o9. 


PHOSPHATES,  ARSENATES  AND  VANADATES     291 

The  mineral  has  been  found  only  in  tiny  crystalline  grains,  so  that  its 
physical  properties  are  not  well  known.  It  is  bright  yellow  in  color,  and 
is  completely  soluble  in  HNO3.  If  to  the  nitric  acid  solution  hydro- 
gen peroxide  be  added  a  brown  color  will  appear.  Or  if  the  solution 
is  filtered,  made  alkaline  by  ammonia  and  through  it  is  passed  H2S,  a 
garnet  color  will  develop.  If  the  mineral  be  moistened  by  a  drop  of 
concentrated  HC1,  a  rich  brown  color  will  result.  The  addition  of  a  drop 
or  two  of  water  will  change  the  color  to  light  green  or  make  it  disappear. 

Occurrence. — Carnotite  occurs  as  a  yellow  crystalline  powder,  some 
of  which  seems  to  consist  of  minute  crystals  with  an  hexagonal  habit, 
in  the  interstices  between  the  grains  in  sandstones  and  conglomer- 
ates, as  nodules  or  lumps  in  these  rocks,  and  as  coatings  on  the  walls 
of  cracks  in  pebbles  in  the  conglomerates  and  on  pieces  of  silicified 
wood  embedded  in  the  sandstones.  It  is  limited  to  very  shallow 
depths  and  is  apparently  a  deposit  from  ground  water. 

Localities. — Its  principal  known  occurrences  are  in  Montrose,  San 
Miguel,  Mesa  and  Dolores  Counties  in  southwestern  Colorado,  especially 
in  Paradox  Valley,  and  in  adjoining  portions  of  New  Mexico  and  Utah, 
and  in  Rio  Blanco  and  Routt  Counties  in  the  northwestern  portion  of 
Colorado.  At  all  these  places  there  are  large  quantities  of  the  impreg- 
nated rock  but  it  contains  on  the  average  only  about  1.5  per  cent  to 
2  per  cent  of  U3O8.  The  mineral  has  also  been  described  from  Mt. 
Pisgah,  Mauch  Chunk,  Pennsylvania,  and  from  Radium  Hill,  South 
Australia. 

Uses. — The  mineral  is  one  of  the  main  sources  of  radium  and  uranium 
and  is  one  of  the  principal  sources  of  vanadium.  Although  it  contains  a 
notable  quantity  of  uranium,  carnotite  has  little  value  except  as  an  ore 
of  radium  and  vanadium,  because  of  the  few  uses  to  which  uranium  is 
put.  This  metal  is  used  to  some  extent  in  making  steel  alloys  and  in  the 
manufacture  of  iridescent  glazes  and  glass.  Its  compounds  are  used  in 
certain  chemical  determinations,  as  medicines,  in  photography,  as  por- 
celain paint,  and  as  a  dye  in  calico  printing.  The  uses  of  vanadium  have 
been  referred  to  on  p.  273. 

The  principal  value  of  carnotite  depends  upon  its  content  of  radium, 
which  in  the  form  of  the  chloride  is  valued  at  about  $40,000  per  gram 
or  $1,500,000  per  oz.  The  importance  of  radium  as  a  therapeutic  agent 
has  not  been  established;  but  that  its  use  is  wonderfully  helpful  in  many 
diseases  is  beyond  question.  Without  doubt  in  the  near  future  carno- 
tite will  become  the  principal  source  of  radium  in  the  world.  Practically 
the  only  other  source  is  the  pitchblende  (p.  297),  of  Gilpin,  Colorado, 
Cornwall,  England  and  Joachimsthal,  Austria, 


292  DESCRIPTIVE  MINERALOGY 

Production. — Carnotite  has  been  mined  in  San  Miguel  and  Montrose 
Counties,  Colorado,  and  at  several  points  in  eastern  Utah,  but  mainly 
for  the  vanadium  it  contains.  At  present  it  is  being  utilized  as  a  source 
of  radium.  From  Colorado  8,400  tons  of  vanadium  ore,  with  a  value 
of  $302,000,  were  shipped  in  19 11  and  from  New  Mexico  and  Utah  about 
70  tons,  valued  at  $3,500.  Some  of  this,  however,  was  vanadinite. 
Most  of  it  was  exported  and  used  as  a  source  of  vanadium.  However, 
the  uranium  content  of  the  carnotite  mined  was  about  1.1  tons  of  the 
metal.  During  191 2  ore  containing  26  tons  of  uranium  oxide  and  6.7 
grams  of  radium  was  produced.  This  would  have  yielded  n.43  grams 
of  radium  bromide,  valued  at  $52,800.  The  present  price  of  standard 
carnotite  carrying  at  least  2  per  cent  U3O8  and  5  per  cent  V2O5,  is  at  the 
rate  of  $1.25  per  lb.  for  the  former  and  thirty  cents  for  the  latter.  In 
1914  the  selling  price  of  4,294  tons  of  carnotite  ore  containing  87  tons 
of  U3O8  was  $103  per  ton.  At  the  present  time  nothing  is  paid  for  the 
radium  content  of  the  ore,  though  this  is  its  most  valuable  component. 
One  ton  of  ore  containing  1  per  cent  of  U3O8  carries  2.566  milligrams  of 
radium.    The  imports  of  uranium  compounds  during  191 2  were  valued 

at  $14,357- 

HYDRATED  ACID  PHOSPHATES  AND  ARSENATES 

A  number  of  hydrated  acid  phosphates  and  arsenates  are  known  to 
constitute  an  isomorphous  group,  but  only  a  few  of  them  occur  as 
minerals.  Brushiie  is  an  acid  calcium  phosphate  and  pharmacolite  is 
the  corresponding  arsenate.  Both  crystallize  in  the  monoclinic  system 
(prismatic  class).    Neither  is  common. 

Pharmacolite  (HCaAs04  •  2H2O)  occurs  principally  in  silky  fibers,  in 
botryoidal  and  stalactic  masses  and  rarely  in  crystals  with  an  axial 
ratio  .6236  :  1  :  .3548  and  #=83°  13'.  Their  cleavage  is  perfect  par- 
allel to  00 Poo  (010).  The  mineral  is  white  or  gray,  tinged  with  red. 
Its  streak  is  white.  It  is  translucent  or  opaque.  Its  luster  is  vitreous, 
except  on  00  P  00  (dio)  where  it  is  slightly  pearly.  Thin  laminae  are 
flexible.  Its  hardness  is  2-2.5  and  density  2.7.  Its  refractive  indices 
for  yellow  light  are:  a=  1.5825, 18=1.5891,  7=1.5937. 

Before  the  blowpipe  pharmacolite  swells  up  and  melts  to  a  white 
enamel.  The  mineral  gives  the  usual  reactions  for  As,  H2O  and  Ca.  It 
usually  occurs  in  the  weathered  zone  of  arsenical  ores  of  Fe,  Ag  and  Co, 
at  Andreasberg,  Harz;  Joachimsthal,  Bohemia,  and  elsewhere. 


CHAPTER  XV 

THE  COLUMBATES,  TANTALATES  AND  URANATES 

The  rare  metals,  columbium  and  tantalum,  exist  in  a  few  silicates, 
but  their  principal  occurrences  are  as  columbates  and  tantalates  which 
are  salts  of  columbium  and  tantalum  acids,  analogous  to  the  various 
acids  of  sulphur.  The  commonest  compounds  are  salts  of  the  meta- 
acids  IfeQ^Oe  and  H2Ta20e,  the  relations  of  which,  to  the  normal  acids, 
are  indicated  by  the  equation  2H3CbC>4—  2H20=H2Cb20e.  Other  im- 
portant minerals  are  derivatives  of  the  pyroacids  corresponding  to 
H4Ct>207,  or  2H3CDO4— H2O.  The  best  known  ortho  salt  is  ferguson- 
ite,  YCb04,  but  it  is  rare. 

All  the  columbates  yield  a  blue  solution  when  partially  decomposed 
in  H2SO4  and  boiled  with  HC1  and  metallic  tin.  The  tantalates  when 
fused  with  KHSO4  and  treated  with  dilute  HC1  give  a  yellow  solution 
and  a  heavy  white  precipitate,  which,  on  treatment  with  metallic  zinc 
or  tin,  assumes  a  deep  blue  color.  When  diluted  with  water  the  blue 
color  of  the  tantalate  solution  disappears,  while  that  of  the  columbate 
solution  remains. 

The  uranates  are  salts  of  uranic  acid,  H2UO4.  The  only  mineral 
known  that  may  be  a  uranate  is  uraninite,  and  the  composition  of  this 
is  doubtful. 

Columbite  ((Fe-Mn)Nb206)  and  Tantalite  ((FeMn)Ta206) 

These  two  minerals  are  isomorphous  mixtures  of  iron  and  manganese 
columbates  and  tantalates.  The  name  columbite  is  applied  to  the  mix- 
ture that  is  composed  mainly  of  the  columbates,  and  tantalite  to  that 
which  is  principally  a  mixture  of  tantalates.  When  the  tantalite  is 
composed  almost  exclusively  of  the  manganese  molecule,  it  is  known  as 
manganotantalitei  Tin  and  tungsten  are  frequently  found  in  both  min- 
erals. 

Their  crystals  are  orthorhombic,  with  a  :  b  :  £=.8285  :  1  :  .8898  for 
the  nearly  pure  columbium  compound,  and  .8304  :  1  :  .8732  for  the 
nearly  pure  tantalum  compound.  Both  form  short  prismatic  crystals 
containing  many  faces,  among  the  most  prominent  being  the  three 
pinacoids;  various  prisms,  notably  00  P(no),  00  P^^o)  and  00  P6(i6o), 

293 


294 


DESCRIPTIVE  MINERALOGY 


and  the  domes  2P  &>  {201)  and  JP  06  (012)  (Fig.  161).  The  most  promi- 
nent pyramids  are  P(in)  and  P3(i33)-  Twins  are  not  uncommon, 
with  2P00  (201)  the  twinning  plane.  The  angle  110A1T0  for  colum- 
bite=79°  17'. 

Both  minerals  are  usually  opaque,  black  and  lustrous,  and  occasion- 
ally iridescent,  though,  in  some  instances,  they  are  translucent  and 
brown.    Their  streak  is  dark  red  or  black.    Their  cleavage  is  distinct 
parallel  to  00  P  66  (100),  fracture  uneven  or  conchoidal,  their  hardness 
6  and  their  specific  gravity 
between    5.3    and    7.3,  in- 
creasing with  the  propor- 
tion of  the  tantalum  mole- 
cules   present.      They    are 
both    infusible   before  the 
blowpipe.   Some  specimens 
exhibit  weak  radioactivity. 
When  columbite  is  de- 
composed  by   fusion   with 
KOH  and  dissolved  in  HC1 
and    H2SO4,   the   solution 
1  turns  blue  on  the  addition 
of  metallic  zinc.    The  min- 
eral is  also  partially  decom- 
posed when  evaporated  to 
dryness  with  H2SO4,  forming  a  white  compound  that  changes  to  yellow. 
When  this  residue  is  boiled  with  HCI  and  metallic  zinc  a  blue  solution 
results.    The  mineral  also  gives  reactions  for  iron  and  manganese. 

Tantalite  is  decomposed  upon  fusion  with  KHSO4  in  a  platinum 
spoon,  or  on  foil.  This  when  heated  with  dilute  HCI  yields  a  yellow 
solution  and  a  heavy  white  powder.  Upon  addition  of  metallic  zinc,  a 
blue  color  results  and  this  disappears  on  dilution  with  water.  In  the 
microcosmic  salt  bead  tantalite  dissolves  slowly,  giving  reactions  for  iron 
and  manganese.  When  treated  with  tin  on  charcoal  the  bead  turns 
green. 

The  two  minerals  may  easily  be  confused  with  black  tourmaline 
(p.  434),  ilmenite  (p.  462)  and  wolframite.  From  tourmaline,  they  are 
distinguished  by  crystallization,  high  specific  gravity  and  luster;  from 
wolframite  by  their  less  perfect  cleavage  and  by  the  reaction  with 
aqua  regia  (see  p.  259);  from  ilmenite  by  the  test  for  titanium, 

Occurrence,  Origin  and  Localities. — Both  minerals  occur  in  veins  of 
coarse  granite  and  probably  have  a  pneumatolytic  origin. 


Fits. 


1  .—Columbite  Crystals  with  «P«,  1 
«=P«,o.o  (6);  «.p,  iio(m);  "Pi.? 
°pl  .  73°  W);  »P3,  130  is);  iP«=,  1 
P,  in  (o)  and  P3,  133  («)■ 


COLUMBATES,  TANTALATES  AND  URANATES    295 

i 

Columbite  is  found  in  granite  veins  at  Bodenmais,  Bavaria;  Tam- 
mela,  in  Finland;  near  Limoges,  France,  with  tantalite;  near  Miask, 
in  the  Ilmen  Mountains,  Russia,  with  samarskite;  and  at  Ivigtut,  in 
Greenland.  In  the  United  States  it  is  found  at  Standish  and  Stone- 
ham,  in  Maine;  at  Acworth,  in  New  Hampshire;  at  Haddam,  in  Con- 
necticut; at  Amelia  Court  House,  Virginia;  with  samarskite  in  the  mica 
mines  in  Mitchell  County,  North  Carolina;  in  the  Black  Hills,  South 
Dakota,  and  at  a  number  of  other  points  in  New  England  and  the  Far 
West. 

Tantalite  is  found  at  many  of  the  localities  for  columbite  and  also 
at  several  other  places  in  Finland;  near  Falun,  in  Sweden;  in  Yancy 
County,  North  Carolina,  and  in  Coosa  County,  Alabama. 

Uses. — At  the  present  time  columbium  and  its  compounds  have  no 
commercial  uses.  Tantalum,  however,  is  employed  in  the  manufacture 
of  filaments  for  certain  types  of  incandescent  lamps.  Since,  however, 
about  20,000  filaments  may  be  made  from  a  single  pound  of  the  metal  the 
market  for  tantalum  ores  is  very  limited. 

Samarskite  and  Yttrotantalite 

These  two  minerals  may  be  regarded  as  isomorphous  mixtures  of 
salts  of  pyrocolumbic  and  pyrotantalic  acids,,  in  which  the  bases  are 
yttrium,  iron,  calcium  and  uranyl. 

Samarskite,  according  to  this  view,  is  approximately 

Y2(Ca-Fe.U02)3(Nb207)3 

and  yttrotantalite  the  corresponding  tantalate.  Yttrium  and  iron  are 
the  principal  bases,  but  there  are  also  often  present  erbium,  cerium, 
tungsten  and  tin. 

Analyses  made  by  Rammelsberg  and  quoted  by  Dana  give  some  idea 
of  the  complexity  of  the  compounds: 


Density 

Ta205    Nb205     W03     Sn02    Ti02*         Y2O3 

Er203 

I-  5-4*5 

46.25       12.32       2.36      1. 12       10.52 

6.71 

n.  5.839 

14.36      41.07                     .16       .56           6.10 

10.80 

in.  5.672 

55  34                      .22       1.08          8.80 

382 

Ce203t 

U02               FeO           CaO           H2O 

Total 

I.  2.22 

1. 61               3.80           5.73           6.31 

98-95 

II.  2.37 

10.90             14.61            .... 

100.93 

ni.  433 

11.94             1430            

99  83 

1.  From  Itterby,  Sweden.       II.  From  North  Carolina.       IH.  From  Miask 

.  Russia. 

* 

Including  Si02.                   t  Including  Di303  and  LajOj. 

296  DESCRIPTIVE  MINERALOGY 

The  first  of  these  three  minerals  has  been  called  yttrotantalite  and 
the  other  two  samarskite.  If  the  first  is  weathered,  as  seems  probable 
from  the  presence  of  over  six  per  cent  of  water,  the  three  may  constitute 
members  of  an  isomorphous  series  with  the  third  representing  the  nearly 
pure  columbate  (samarskite),  the  first  a  compound  in  which  the  tantalate 
molecule  is  in  excess  (yttrotantalite),  and  the  second  an  intermediate 
compound  which  contains  both  the  tantalum  and  columbium  molecules, 
with  the  latter  predominating. 

With  more  accurate  analyses  the  great  complexity  of  these  compounds 
becomes  even  more  apparent.  Hillebrand  has  given  the  following  report 
of  his  analysis  of  a  samarskite  from  Devil's  Head  Mountain,  near  Pike's 
Peak,  Colorado,  which  shows  the  futility  of  attempting  to  represent  its 
composition  by  a  chemical  formula: 

Pitch-black  Black  Weathered 

Variety  Variety  Variety 

Tajd 27.03  28.II  19-34 

Cb2Oft 27.77  26.16  27.56 

WOi 2.25  2.08  5.51 

SnO« 95  1.09  .82 

ZrOj 2 .  29  2 .  60  3 .  10* 

UO» 4.02  4.22                   

UOj ...  6 .  20 

ThO* 3.64  3.60  3.19 

Ce808 54  .49  .41 

(La,Di)208 1.80  2.12  1.44 

Erj02 10.71  10.70  9.82 

Y2O3 6.41  5.96  5.64 

Fe*0, 8.77  8.72  8.90 

FeO 32  .35  .39t 

MnO 78  .75  1 

ZnO 05  .07  I'77 

PbO 72  .80  1.07 

CaO 27  .33  1. 61 

MgO ...  .11 

K*° 17  .13  \    , 

(Na,Li),0 24  .17  /'3 

H2O 1.58  1.30  3.94 

F ?  ?  ? 

Total 100.31  99-75  100.18 

Sp.gr 6.18  6.12  5.45 

♦With  Ti02.  t  Or  0.74  UO*. 


COLUMBATES,  TANTALATES  AND  UEANATES     297 


Both  samarskite  and  yttrotantalite  are  orthorhombic,  with  an  axial 
ratio  for  samarskite  of  .5456  :  1  :  .5178,  and  for  yttrotantalite,  .5411  : 
1  :  1. 1330.  They,  however,  more  commonly  occur  massive  and  in 
flattened  grains  embedded  in  rocks.  Their  crystals  are  prismatic  in 
the  direction  of  the  c  or  the  b  axis.  Their  most  prominent  forms  are 
00  P*  (100),  00  P  66  (010)  and  P  66(101)  (Fig.  162).  Less  prominent 
but  fairly  common  are  *>P2(i2o),  ooP(no),  P(iu)  and  3Pf(23i). 
The  angle  110A1T0  for  samarskite  is  570  14' 
and  for  yttrotantalite  560  50'. 

The  cleavage  of  both  minerals  is  indistinct 
parallel  to  00  P  06  (010).  Their  fracture  is 
conchoidal.  Both  are  brittle.  The  hardness  of 
samarskite  is  5-6,  its  density  about  5.7,  its 
luster  vitreous,  its  color  velvety  black  and  its 
streak  reddish  brown.  Yttrotantalite  is  a  little 
softer  (5-5.5).  Its  specific  gravity  is  5.5-5.9, 
its  luster  submetallic  to  vitreous,  its  color  black,  Fig.  162.— Samarskite  Crys- 
brown,  or  yellow,  and  its  streak  gray  to  color-  **!  with  °°  P  * ,  100  (a); 
less.  Samarskite  is  opaque  and  yttrotantalite 
opaque  or  translucent. 

The  reactions  of  the  minerals  vary  with 
their  composition.  They  always  yield  the 
blue  solution  test  for  tantalum  or  columbium,  and  most  specimens  react 
for  Mn,  Fe,  Ti  and  U.  The  reaction  for  uranium  is  an  emerald  green 
bead  with  microcosmic  salt  in  both  reducing  and  oxidizing  flame. 

They  are  distinguished  from  columbite  and  tantalite  by  the  form  of 
their  crystals. 

Occurrence. — The  two  minerals,  like  columbite  and  tantalite,  are 
found  principally  in  pegmatite  veins  and  in  many  of  the  same  localities. 
Yttrotantalite  occurs  mainly  at  Ytterby  and  near  Falun,  in  Sweden,  and 
samarskite,  near  Miask  in  the  Ilmen  Mountains,  Russia.  In  the  United 
States  the  last-named  mineral  is  sometimes  found  in  large  masses  in  the 
mica  pegmatites  of  Mitchell  County,  North  Carolina. 

Uses. — Neither  mineral  is  at  present  of  any  commercial  value.  They 
are,  however,  extremely  interesting  as  the  source  of  many  of  the  rare 
elements,  and,  especially,  as  a  possible  source  of  radium  and  closely 
related  substances. 

Uraninite 

Uraninite,  or  pitchblende,  like  the  other  compounds  containing  the 
element  uranium,  is  of  doubtful  composition.     It  contains  so  many 


00 Poo,  010  (b);  00 p, 
no  (m);  00  P2,  120  (A); 
P«,  101  («);  P,ni  (p); 
3Pf,  231  (»). 


298  DESCRIPTIVE  MINERALOGY 

different  components  that  a  correct  conception  of  its  character  is  almost 
impossible  to  grasp.  The  mineral  is  particularly  interesting  because  it 
always  contains  a  trace  of  radium,  of  which  it  is  an  important  com- 
mercial source  at  the  present  time. 

Analyses  of  crystallized  material  (I)  from  Branchville,  Conn., 
and  from  Annerod  (II),  Norway,  gave  the  following  results: 

U03  U02      TI1O2       PbO   FejjOa    CaO   H20       He       Insol. 

I.  21.54      64.72      6.93      4.34       .28       .22       .67      Und.  .14 

II.30.63      46.13      6.00      9.04       .25       .37       .74         .17        442 

with  small  quantities  also  of  Z1O2,  CeCfe,  La203,  Di203,  Y2O3,  Er2C>3, 
MnO,  Alkalies,  Si02  and  P2O5.  These  analyses  are  interpreted  as  indi- 
cating that  the  mineral  is  a  uranium  salt  of  uranic  acid,  UQ2(OH)2,  or 

H2UO4,  thus   U  -^>n  ,  or  U3O8,  in  which  Pb  replaces  the  U  in 

\)>U02 
part,  and  TI1O2  the  UO2.    Radium  is  found  in  most  specimens  and 
helium  in  nearly  all. 

Several  varieties  are  recognized,  the  distinctions  being  based  largely 
upon  chemical  differences. 

Broggerite  has  UO3  to  other  bases  as  1  :  1. 

Cleveite  and  nhenile  contain  9  per  cent  to  10  per  cent  of  the  yttria 
earths. 

Pitchblende  is  possibly  an  amorphous  uraninite  containing  a  very 
little  thoria  and  much  water.  Its  specific  gravity  is  often  as  low  as  6.5, 
due  probably  to  partial  alteration. 

Uraninite  crystallizes  in  the  isometric  system  in  octahedrons,  and  in 
combinations  of  O(in),  00  O(no),  and  00  O  00  (100).  Crystals  are  rare, 
however,  the  material  usually  occurring  in  crystalline  masses  and  in 
botyroidal  groups. 

The  mineral  is  gray,  brown  or  black  and  opaque.  Its  streak  is 
brownish  black,  gray  or  olive  green.  Its  luster  is  pitch-like  or  dull.  Its 
fracture  is  uneven  or  conchoidal.  It  is  brittle,  its  hardness  is  5.5  and 
density  9-9.7.    Like  the  other  uranium  minerals  it  is  radioactive. 

Before  the  blowpipe  uraninite  is  infusible.  Some  specimens  color 
the  flame  green  with  copper.  With  borax  it  gives  a  yellow  bead  in  the 
oxidizing  flame,  turning  green  in  the  reducing  flame.  All  specimens  give 
reactions  for  lead  and  many  for  sulphur  and  arsenic.  The  mineral  is 
soluble  in  nitric  and  sulphuric  acids,  with  slight  evolutions  of  helium, 


COLUMBATES,  TANTALATES  AND  URANATES     299 

the  ease  of  solubility  increasing  with  the  increase  in  the  proportion  of 
rare  earths  present. 

Uraninite  is  distinguished  from  wolframite,  samarskite,  columbite  and 
tantalite,  by  lack  of  cleavage,  greater  specific  gravity,  and  differences  in 
crystallization.  From  all  but  samarskite  it  is  also  distinguished  by  the 
reactions  for  uranium  and,  in  the  case  of  most  specimens,  by  the  reac- 
tion for  lead.    It  is  especially  characterized  by  its  pitch-black  luster. 

Occurrence  and  Localities, — Uraninite  occurs  in  pegmatites  and  in 
veins  associated  with  silver,  lead,  copper  and  other  ores.  It  is  found  in 
the  ore  veins  in  Saxony,  Bohemia,  and  in  pegmatites  near  Moss,  Arendal 
and  other  points  in  Norway. 

In  the  United  States  it  occurs  in  pegmatites  at  Middletown  and 
Branchville,  in  Connecticut;  at  the  Mitchell  County  mica  mines, 
North  Carolina;  and  at  Barringer  Hill,  Llano  County,  Texas.  It  is 
also  found  in  large  quantity  near  Central  City,  Gilpin  County,  Colorado, 
where  it  is  associated  with  gold,  galena,  tetrahedrite,  chalcopyrite  and 
other  ore  minerals. 

Production. — Uraninite  has  been  mined  in  small  quantity  in  Colo- 
rado, and  at  Barringer  Hill,  both  as  a  source  of  uranium  and  as  a 
source  of  radium.  In  Cornwall,  England,  and  at  Joachimsthal, 
Austria,  it  is  mined  as  a  source  of  radium.      (See  also  p.  292.) 


CHAPTER  XVI 

THE  SILICATES 

The  silicates  are  salts  of  various  silicon  acids,  only  a  few  of  which 
are  known  uncombined  with  bases.  The  silicates  include  the  commonest 
minerals  and  those  that  occur  in  largest  quantity.  They  make  up  the 
greater  portion  of  the  earth's  crust,  forming  most  of  the  igneous  rocks 
and  a  large  portion  of  vein  fillings.  In  number,  the  silicates  exceed  all 
other  mineral  compounds,  but  because  of  their  stability  they  are  of  very 
little  economic  importance.  A  few  are  used  as  the  sources  of  valuable 
substances,  and  their  aggregates,  the  silicious  rocks,  are  utilized  as 
building  stones,  but,  on  the  whole,  they  are  of  little  commercial  value. 
Since,  however,  they  occur  in  good  crystals  and  their  material  is  trans- 
parent in  thin  sections  so  that  it  can  easily  be  studied  by  optical  methods, 
they  are  of  great  scientific  importance.  Much  of  the  progress  made  in 
crystallography  has  been  accomplished  through  the  study  of  these  com- 
pounds. 

Although  the  salts  of  the  silicic  acids  are  very  numerous  and  most  of 
them  are  very  stable  toward  the  ordinary  reagents  of  the  laboratory, 
the  acids  from  which  they  are  derived  are  only  imperfectly  known. 
The  only  one  that  has  been  prepared  in  the  pure  state  is  the  compound 
H2Si03.  This  occurs  as  a  gelatinous  (colloidal)  white  substance  which 
rapidly  loses  water  upon  drying  and  probably  breaks  up  into  a  number 
of  other  compounds  which  are  also  acids,  containing,  however,  a  larger 
proportion  of  silicon  in  the  molecule  than  that  in  the  original  compound. 
When  the  tetrafluoride,  or  the  tetrachloride,  of  silicon  is  decomposed  by 
water,  the  principal  product  is  the  acid  referred  to  above,  but  in  addition 
to  this  there  is  probably  formed  also  the  compound  I^SiCU  or  Si(OH)4, 
which  is  the  ortho  acid.  Some  silicates  are  salts  of  these  acids.  Others 
are  salts  of  the  acids  containing  a  larger  proportion  of  silicon.  In  most 
cases,  however,  these  acids  may  be  regarded  as  belonging  to  a  series  in 
which  the  members  are  related  to  one  another  in  the  same  manner  as 
are  normal  sulphuric,  common  sulphuric  and  pyrosulphuric  acids.  Nor- 
mal sulphuric  acid  is  HeSOe.  By  abstraction  of  2H2O  the  compound 
H2SO4,  or  ordinary  sulphuric  acid,  results.  If  from  two  molecules  of 
H2SO4,  one  molecule  of  HoO  is  abstracted,  H2S2O7,  or  pyrosulphuric 
acid,  is  left.    In  the  same  manner  all  of  the  silicic  acids  may  be  regarded 

300 


SILICATES  301 

as  being  derived  from  normal  silicic  acid  Si(OH)4  or  H4Si04  by  the  ab- 
straction of  water,  thus: 

Orthosilicic  acid  is      RtSiC^, 
Metasilicic  acid  is       H4Si04  —  H2O  or  IfcSiOa, 
Diorthosilicic  acid  is  2H4Si()4— H2O  or  HeSi207, 
Dimetasilicic  acid  is    2H2Si03— H2O  or  H2S12O5, 
Trimetasilicic  acid  is  3H2Si03— H2O  or  HiS^Og. 

The  compounds  containing  more  than  one  silicon  atom  in  the  molecule 
are  known  as  polysilicates.  The  salts  of  metasilicic  acid  are  meta- 
silicates. 

Many  attempts  have  been  made  to  discover  the  chemical  structure 
of  the  comparatively  simple  silicates  and  several  proposals  have  been 
offered  to  explain  the  great  differences  often  observed  in  the  properties 
of  silicates  with  the  same  empirical  formula;  but  no  explanation  of  these 
differences  has  thus  far  proved  satisfactory.  The  silicates  are  so  very 
stable  under  laboratory  conditions,  and,  when  they  are  decomposed, 
their  decomposition  products  are  so  difficult  to  study,  that  it  has  been 
impossible  to  determine  their  molecular  volumes  or  to  understand  their 
substitution  products.  We  are  thus  driven  to  ascribe  many  of  the 
anomalies  in  their  composition  to  solid  solutions,  to  absorption  phenom- 
ena, and  to  the  isomorphous  mixing  of  compounds,  some  of  which  do 
not  exist  independently. 

There  are  many  silicates,  moreover,  which  cannot  be  assigned  to  any 
of  the  simple  acids  mentioned  above,  but  which  probably  must  be 
regarded  as  salts  of  very  much  more  complex  acids.  Others  are  pos- 
sible salts  of  aluminosilicic  acids  in  which  aluminium  functions  in  the 
acid  portions.  Thus,  albite  is  usually  regarded  as  a  trisilicate,  NaAlSiaOs, 
and  anorthite  as  an  orthosilicate,  CaAl2(Si04)2.  But  the  two  substances 
are  completely  isomorphous,  and  for  this  reason  it  is  thought  that  they 
must  be  salts  of  the  same  acid.  If  we  assume  an  aluminosilicic  acid  of 
the  formula  HsA^Os,  albite  may  be  written  (NaSi)AlSi20s,  and  anor- 
thite (CaAl)AlSi208.  The  two  minerals  thus  become  salts  of  the  same 
acid  and  their  complete  isomorphism  is  explained.  The  relations  that 
exist  among  many  silicates  might  be  better  understood  on  the  assump- 
tion that  they  are  salts  of  complex  silicic  and  of  aluminosilicic  acids 
than  on  the  assumption  that  they  are  salts  of  simpler  acids,  as  is  now  the 
case.  But,  since  it  has  been  impossible  to  isolate  the  acids  and  study 
them  we  are  not  certain  as  to  their  character.  It  is,  therefore,  believed 
best  to  represent  most  silicates  as  salts  of  the  simplest  acids  possible, 
consistent  with  their  empirical  compositions  as  determined  by  analyses. 


302  DESCRIPTIVE  MINERALOGY 

As  in  the  case  of  salts  of  other  acids  there  are  silicates  that  contain 
hydrogen  and  oxygen  in  such  relations  to  their  other  components  that 
when  heated  they  yield  water.  In  some  cases  this  water  is  driven  off  at 
a  comparatively  low  temperature  and  the  residue  of  the  compound  re- 
mains unchanged.  A  compound  of  this  kind  is  usually  called  a  hydrate 
or  the  compound  is  said  to  contain  water  of  crystallization.  In  other 
cases  a  high  temperature  is  necessary  to  drive  off  water,  and  the  com- 
pound breaks  up  into  simpler  ones.  In  these  instances  the  water  is 
said  to  be  combined.    The  compound  is  usually  basic. 

In  the  descriptions  of  the  silicates  the  order  in  which  the  minerals  are 
discussed  is  that  of  increasing  acidity,  i.e.,  increasing  proportion  of  the 
SiCfe  group  present  in  the  molecule.  This  order,  however,  is  not  fol- 
lowed rigorously.  The  members  of  well  defined  groups  of  closely  related 
minerals  are  discussed  together  even  if  their  acidity  varies  widely. 
Nearly  all  the  silicates  are  transparent  or  translucent  and  all  are  elec- 
trical insulators. 

THE  ANHYDROUS  ORTHOSILICATES 

NORMAL  ORTHOSILICATES— R^SiC^ 
OLIVINE  GROUP   (R"flSi04).    R"  -  Mg,  Fe,  Mn,  Zn 

The  members  of  the  olivine  group  are  normal  silicates  of  the  metals 
Mg,  Fe,  Mn  and  Zn.  They  constitute  an  isomorphous  series  crystalliz- 
ing in  the  holohedral  division  of  the  orthorhombic  system  (rhombic  bi- 
pyramidal  class).  The  most  common  member  is  the  magnesium-iron 
compound  (Mg-Fe)2Si04,  olivine,  or  chrysot.le,  from  which  the  group 
gets  its  name.  The  members  with  the  simplest  composition  are  for- 
sterile  (Mg2Si04),  fayalitc  (Fe2Si04)  and  tephroite  (Mn2Si04).  The 
others  are  isomorphous  mixtures  of  these,  with  the  exception  of  three 
rare  minerals,  of  which  one,  monticeUUe,  is  a  calcium  magnesium  silicate, 
another,  titanolivine,  contains  Ti  in  place  of  a  part  of  the  Si,  and  the 
other,  roepperite,  contains  some  Zn2Si04.  Most  of  them  are  formed 
by  crystallization  from  molten  magmas. 

Crystals  of  all  the  members  of  the  group  are  prismatic  and  all  have 
nearly  the  same  habit.  They  are  often  flattened  parallel  to  one  of  the 
pinacoids,  oo  P  06  (010)  or  00  P  60  (100).  The  axial  ratios  of  the  com- 
moner members  are  as  follows: 

Forsterite  a  :  b  :  £=.4666  :  1  :  .5868.  The  angle  no  A  i7o=5o°  2' 

Olivine  =.4658  :  1  :  .5865.  The  angle  no  A  11*0=49 °  57' 

Tephroite  =.4600  :  1  :  .5939.  The  angle  no  A  1^0=49°  24' 

Fayalite  =4584  :  1  :  .5793.  The  angle  noAiio=49°  *5' 


ANHYDROUS  ORTHOSILICATES 


303 


m 


Crystals  of  olivine  are  usually  combinations  of  some  or  all  of  the  following 
forms:      oo  P  66  (ioo),     oo  P  oo  (oio), 
oP(ooi),        ooP(no),       ooP2(i2o), 

Poo  (oil),       2Poo(o2l),       Pob(lOl), 

P(iii)  and  2P2(i2i)  (Fig.  163). 
The  crystals  of  fayalite  are  usually 
more  tabular  than  those  of  olivine, 
but  forsterite  and  tephroite  crystals 
have  nearly  the  same  forms.  The 
cleavage  of  all  is  distinct  parallel 
to  00  P60  (010),  less  distinct  parallel 
to  00  P  66  (100)  in  olivine,  and  par- 
allel to  oP(ooi)  in  fayalite. 

The  compositions  of  the  pure  Mg, 
Mn,  and  Fe  molecules  are: 


Fig.  163. — Olivine  Crystals  with 
00 P,  no  (m);  00 Poo,  010  (6); 
oP,  001  (c);  2P 00  ,  021  (k);  oo  P2 , 
120  (*),P  00  ,  xoi  (d)  and  P,  in  (e). 


MgO. 
MnO. 
FeO. 
Si02. 


Mg2Si04 

-  57-x 


42.9 


Mn2Si04 
70.25 

29  -75 


Fe2Si04 


70.6 
29.4 


All  natural  crystals,  however,  contain  some  of  all  the  metals  indicated 
and,  in  addition,  many  specimens  contain  also  a  determinable  quantity 
of  CaO  and  traces  of  other  elements. 


Forsterite,  Olivine  and  Fayalite  (Mg2Si04  -  (Mg  •  Fe)2Si04  - Fe2Si04) 

The  composition  of  olivine  naturally  depends  upon  the  proportion 
of  the  forsterite  and  fayalite  molecules  present  in  it.  When  the  propor- 
tion of  FeO  exceeds  24  per  cent,  the  variety  is  known  as  hyalosiderite. 
A  few  typical  analyses  are  quoted  below: 


MgO 

FeO 

CaO 

AI2O3 

Si02 

Total 

Sp.  Gr. 

I.  51.64 

5-oi 

1.08 

42 

42.30  . 

100.45 

3.261 

II.  50.27 

8.54 

•      a      •      ■ 

•      •      • 

41.19 

100.00 

III.  48.12 

n. 18 

.12 

a      •      • 

40.39 

99.81 

3-294 

IV.  39.68 

22.54 

a      •      ■      • 

a       a      • 

37.17 

99  39 

• 

I.  From  masses  enclosed  in  Vesuvian  lava. 
II.  Concretion  in  basalt  near  Sasbach,  Kaisers tuhl. 

III.  Grains  from  glacial  debris,  Jan  Mayen,  Greenland. 

IV.  Grains  from  coarse-grained  rock,  near  Montreal,  Canada. 


a 

P 

7 

I. 6319 

1.6519 

I . 6698 

1 . 6674 

1 . 6862 

1 • 70S3 

1.8236 

1 . 8642 

1.8736 

304  DESCRIPTIVE  MINERALOGY 

In  addition,  there  are  often  also  present  small  quantities  of  Ni,  Mn, 
and  Ti. 

Forsterite,  olivine  and  fayalite  are  usually  yellow  or  green  in  color 
and  have  a  vitreous  luster.  Forsterite  is  sometimes  white  and  olivine 
often  brown.  All  three  minerals  become  brown  or  black  on  exposure 
to  the  air.  All  are  transparent  or  translucent.  Their  streak  is  colorless 
or  yellow.  The  fracture  of  olivine  is  conchoidal.  In  the  other  two 
minerals  it  is  uneven.  Their  hardness,  density  and  refractive  indices 
for  yellow  light  are  as  follows: 

Hardness      Sp.  Gr. 

Forsterite 6-7      3 .  21-3 .  33 

Olivine 6.5-7      3-27"3-37 

Fayalite 6.5      4.00-4.14 

Before  the  blowpipe  most  olivines  and  forsterites  whiten  but  are  in- 
fusible. Their  fusion  temperatures  are  between  13000  and  14500, 
decreasing  with  increase  in  iron.  Fayalite  and  varieties  of  olivine  rich 
in  iron  fuse  to  a  black  magnetic  globule.  All  three  minerals  are  decom- 
posed by  hydrochloric  and  sulphuric  acids  with  the  separation  of  gelat- 
inous silica;  the  iron-rich  varieties  are  decomposed  more  easily  than 
those  poor  in  iron. 

The  minerals  are  characterized  by  their  color  and  solubility  in 
acids. 

Both  fayalite  and  olivine  alter  on  exposure  to  the  air,  the  former 
changing  to  an  opaque  mixture  of  Fe203  and  Si02,  or  to  the  fibrous 
mineral  anthophyllite  ((Mg*Fe)Si03),  and  olivine  to  a  mixture  of 
iron  oxides  and  fibrous  or  scaly  gray  or  green  serpentine  (H4Mg3Si20Q). 
In  other  cases,  under  metamorphic  conditions,  the  alteration  is  to  a 
red  lamellar  mineral  (iddingsite)  which  may  be  a  form  of  serpentine, 
or  to  magnesite,  or  to  the  silicate,  talc.  Other  kinds  of  alteration  of 
this  mineral  have  also  been  noted  but  those  described  are  the  most 
common. 

Syntheses. — The  members  of  the  olivine  series  have  been  produced 
by  fusing  together  the  proper  constituents  in  the  presence  of  magnesium 
and  other  chlorides.  They  are,  moreover,  present  in  many  furnace 
slags  where  they  have  been  made  in  the  process  of  ore  smelting. 

Occurrence. — Olivine  occurs  as  an  original  constituent  of  basic  igneous 
rocks  and  as  a  metamorphic  product  in  dolomitic  limestones.  It  is 
found  also  in  the  form  of  rounded  grains  in  some  meteoric  irons.  Fayalite 
occurs  in  acid  igneous  rocks,  especially  where  affected  by  pneumatoiytic 


ANHYDROUS  ORTHOSILICATES  305 

action,  and  forsterite  in  dolomitic  rocks  when  they  have  been  meta- 
morphosed by  the  action  of  igneous  rocks. 

Localities. — Members  of  the  olivine  group  occur  in  the  basaltic  lavas 
of  many  volcanoes — as  those  of  the  Sandwich  Islands;  in  the  limestone 
inclusions  in  the  lava  of  Mt.  Somma,  near  Naples;  in  various  basic 
rocks  in  Vermont  and  New  Hampshire  and  at  Webster,  N.  C.  At  the 
latter  place  granular  aggregates  of  almost  pure  olivine  constitute  great 
rock  masses  known  as  dunite, 

Fayalite  is  found  in  the  rhyolites  of  Mexico,  the  Yellowstone  Park 
and  elsewhere,  and  in  coarse  granite  at  Rockport,  Mass.,  and  in  the 
Mourne  Mountains,  Ireland. 

Forsterite  occurs  in  limestone  enclosures  in  the  lava  of  Mt.  Somma 
and  at  limestone  contacts  with  igneous  rocks  at  Bolton,  Roxbury,  and 
Littleton,  Mass.,  and  elsewhere. 

Uses  and  Production. — The  only  member  of  the  group  that  is  of  any 
economic  importance  is  a  pale  yellowish  green  transparent  olivine,  which 
is  used  as  jewelry  under  the  name  of  "  peridot."  Gem  material  is  found 
at  Fort  Defiance  and  Rice,  in  Arizona,  scattered  loose  in  the  soil.  The 
little  grains  came  from  a  basic  volcanic  rock.  The  amount  produced  in 
the  United  States  during  191 2  was  valued  at  about  $8,100. 

Tephroite  (Mn2Si04) 

Although  tephroite  is  regarded  as  the  manganese  silicate  it  nearly 
always  contains  some  of  the  forsterite  molecule. 

Analyses  of  brown  (I),  and  red  (II),  varieties  from  Sterling  Hill 
gave: 

MnO        FeO     MgO       CaO      ZnO       Loss       Si02       Total 

I.  52.32        1.52        7.73      1.60      5.93        .28       30.55        99.93 

II.47.62  .23      14.03        .54      4.77        .35        31.73        99.27 

The  mineral  is  gray,  brown  or  rose-colored  and  transparent  or 
translucent.  Its  streak  is  nearly  colorless.  It  is  rarely  found  in  crys- 
tals. Its  hardness  is  about  6  and  its  density  4.08.  It  is  strongly 
pleochroic  in  reddish,  brownish  red  and  greenish  blue  tints.  Its  inter- 
mediate refractive  index  for  yellow  light  =  about  1.80. 

It  is  fusible  with  difficulty  (fusing  temperature  =1200°),  and  is  sol- 
uble in  HC1  with  separation  of  gelatinous  silica.  It  is  distinguishable 
from  other  like-appearing  minerals  by  its  difficult  fusibility  and  its 
reaction  with  HC1. 

Syntheses. — Crystals  of  the  mineral  have  been  made  by  fusing  to- 
gether Si02  and  Mn02  in  the  proportion  of  1  :  2,  and  by  long-continued 


306  DESCRIPTIVE  MINERALOGY 

heating  of  MnCk  and  Si02  in  an  atmosphere  of  moist  hydrogen  or  carbon 
dioxide. 

Localities. — Tephroite  occurs  at  Mine  Hill  and  Sterling  Hill,  near 
Franklin,  N.  J.,  where  it  is  associated  with  franklinite,  zincite  and 
troostite.  It  is  found  also  at  Pajsberg  in  Sweden  with  other  man- 
ganese minerals  and  magnetite,  and  at  Langban,  in  Wermland, 
Sweden. 

Uses. — The  mineral  is  of  little  commercial  value.  It  is  separated 
with  other  manganese  minerals  from  the  zinc  ore  of  Franklin,  N.  J.,  and 
is  smelted  with  these  in  the  production  of  spiegeleisen. 

WILLEMITE  GROUP  (R,"SiO*).    R"=Zn,  Mn. 

The  willemite  group  comprises  the  two  minerals  -willemite  (Zn2Si04) 
and  troostite  ((Zn-Mn^SiCU),  of  which  the  latter  is  rare.  Willemite 
occurs  in  small  quantity  only,  but  troostite  is  an  important  source  of 
zinc  at  the  Franklin  locality  in  New  Jersey.  Both  minerals  are  found  in 
crystals. 

Willemite  and  troostite  crystallize  in  the  rhombohedral  hemihedral 
division  of  the  hexagonal  system  (ditrigonal  scalenohedral  class),  with 
the  axial  ratios 

Willemite    a  :  c=i  :  0.6698 
Troostite  =  1  :  0.6698 

Willemite  and  Troostite  (Zn2Si04- (Zn  Mn)2Si04) 

Willemite  and  troostite  occur  massive,  in  grains,  and  in  simple  crys- 
tals. 

The  theoretical  composition  of  willemite  is  Si02=  27.04  and  ZnO 
=  72.96,  but  nearly  all  natural  crystals  contain  traces  of  other  elements. 
When  a  noticeable  quantity  of  manganese  is  present,  the  compound 
is  troostite.    Several  analyses  are  quoted  below: 

SiO* 

Willemite  from  Stolberg,  Germany 26 .  00 

Willemite  from  Greenland 27 .  86 

White  troostite  from  Franklin,  N.  J..  .  .      27. 20 
Dark  red  troostite  from  Franklin,  N.  J. .     27. 14 

The  crystals  of  willemite  exhibit  the  forms  00  R(ioTo),  00  P 2(1 120), 
oR(oooi),  111(3034)  and  —  £R(oil2)(Fig.i64).  Twins,  with |P2 (3.3.6. 10) 
as  the  twinning  planes,  are  rare.  The  crystals  of  troostite  are  even 
more  simple,  with    ooP2(ii2o)  and  R(ioTi),  usually  the  only  forms 


ZnO 

MnO 

FeO 

Total 

72.91 

•   •  *   • 

•  35 

100.16 

7I-SI 

•   •  •   ■ 

37 

99-74 

65.82 

6-97 

23 

100.22 

64.38 

6.30 

1.24 

99.06 

ANHYDROUS  ORTHOSILICATES 


307 


present,  though  —  £R(oiT2),  —  |R(°332)  and  R3(2i3i)  are  also  occa- 
sionally found.  The  angle  10I1 A  1101  =  63°  59'.  The  cleavage  of 
willemite  is  distinct  parallel  to  oP(oooi),  and  of  troostite  distinct 
parallel  to  ooP2(ii2o),  and  less  perfect  parallel  to  R(ioTi)  and 
oR(oooi). 

Willemite  is  colorless,  yellow,  brown  or  blue.    Troostite  is  green, 
yellow,  brown  or  gray.    The  colored  varieties  of  both  minerals  are 
translucent.    Colorless  willemite  is  transparent.    Both  minerals  are 
vitreous  in  luster.     Their  hardness  is  between 
5  and  6  and  density  between  3.9  and  4.3.    The 
refractive  indices  of  willemite  for  yellow  light 
are:    u>  =1.6931,  €=1.7118. 

Both  minerals  glow  when  heated  before  the 
blowpipe  and  are  fused  with  difficulty  (about 
1484 °),  and  both  gelatinize  with  HC1.  Willem- 
ite gives  the  reaction  for  zinc  with  Co(N03)2 
on  charcoal,  and  troostite  gives,  in  addition, 
the  reaction  for  manganese. 

Syntheses. — Willemite  crystals  have  been 
made  by  the  action  of  gaseous  hydrofluo- 
silicic  acid  upon  zinc,  and  by  the  action  of 
silicon  fluoride  on  zinc  oxide  at  cherry-red  temperature. 

Localities  and  Origin. — Willemite  occurs  in  comparatively  small  quan- 
tity at  only  a  few  places,  associated  with  other  zinc  minerals.  In 
America  it  is  found  in  colorless  and  black  crvstals  at  the  Merritt 
Mine  near  Socorro,  New  Mexico,  associated  with  mimetite,  wulfenite, 
cerussite,  barite  and  quartz. 

Troostite  occurs  only  at  Sterling  Hill  and  Franklin  Furnace,  N.  J., 
but  in  such  large  quantity  that  it  constitutes  an  important  proportion 
of  the  zinc  ore  for  which  these  localities  are  noted.  It  is  associated  with 
franklinite  and  zincite.  Both  willemite  and  troostite  are  results  of 
magmatic  processes. 


Fig.  164. — Willemite  Crys- 
tal with  00  P2,  1 1 20  (a); 
R,  ioii  (r)  and  —  JR, 
01 T2  (e). 


Phenacite  (Be2SiC>4) 

The  theoretical  composition  of  the  compound  Be2Si(>4  is  Si04  =  54.47, 
BeO= 45.53.  Many  of  the  analyses  of  phenacite  show  that  it  ap- 
proaches very  closely  to  this.  A  specimen  from  Durango,  Mexico,  for 
example,  is: 


SiO=  54.71,  BeO=45-32,  MgO+CaO=.i4.    Total =  100.17. 


308  DESCRIPTIVE  MINERALOGY 

Phenacite  crystallizes  in  the  rhombohedral  tetartohedral  division  of 
the  hexagonal  system  with  a  :  c=  i  :  1.0661.  It  occurs  in  crystals  pos- 
sessing many  different  types  of  habit  and  with  many  different  combina- 
tions of  forms.  Perhaps  ooP2(iiIo),  ooP(ioTo),  R(ioli),  R3(2i3i) 
and  —  JR(oii2)  are  the  most  common  (Fig.  165).    Interpenetration 

twins  are  common  at  some  localities.  The 
cleavage  is  indistinct  parallel  to  00  P(io7o). 
The  angle  10T1  Alioi  =  630  24'. 

Phenacite  is  colorless  or  white  or  some 
light  shade  of  yellow  or  pink.  It  is  trans- 
parent or  translucent  and  has  a  glassy  luster. 
Its  hardness  is  7.5,  and  density  about  3  and 
the  refractive  indices  for  yellow  light  are: 

Fig.  i6S.-Phenacite  Crystal  »=I-6542,  c- 1.6700.  It  is  infusible  and 
with  00  P2,  1120  (a);   « p,  insoluble   in   acids.    When  heated  with  a 

_  -JP3 little  soda  before  the  blowpipe  it  affords  a 

/  v  2      '  white  enamel.     The  mineral  is  phosphores- 

cent and  pyroelectric. 

Colorless  phenacite  resembles  quartz  and  I.erderiie,  and  the  yellow 
variety  topaz.  It  is  best  distinguished  from  them  by  its  crystalliza- 
tion. 

Syntheses. — Small  crystals  have  been  made  by  the  fusion  of  a  mix- 
ture of  SiOa  and  beryllium  oxide  and  borax,  and  by  melting  together 
beryllium  nitrate,  silica  and  ammonium  nitrate. 

Localities. — Phenacite  occurs  at  the  Emerald  Mines  near  Ekaterin- 
burg in  the  Urals;  near  Fremont,  in  the  Vogesen;  at  Reckingen,  in 
Switzerland;  in  Durango,  Mexico;  near  Pike's  Peak,  at  Topaz  Butte, 
and  at  Mount  Antero,  in  Colorado,  and  at  Greenwood,  in  Maine.  In 
all  cases  the  mineral  is  probably  a  result  of  pneumatolysis. 

Uses, — The  colorless  phenacite  is  used  to  a  slight  extent  as  a  gem. 

GARNET  GROUP 
(R",R'",(Si04)s).    R"=Ca,  Mg,  Fe,  Mn.     R"'=A1,  Fe,  Cr 

The  garnet  group  comprises  a  large  number  of  isomorphous  com- 
pounds, some  of  which  are  very  common.  The  members  nearly  all 
occur  in  distinct  crystals  that  are  combinations  of  isometric  holohedrons 
(hexoctahedral  class).  Many  different  names  have  been  given  to  the 
garnets  and  analyses  show  that  they  possess  very  different  compositions. 
With  the  exception  of  a  few  rare  varieties,  they  can  all,  however,  be 
explained  as  consisting  of  one  of  the  six  molecules  indicated  below,  or  of 


ANHYDROUS  ORT*HOSILICATES 


309 


mixtures  of  them.    The  six  molecules  and  the  names  of  the  garnets 
corresponding  to  them,  together  with  their  densities,  are: 

Ca3Al2(Si04)3    Grossularite  or  Hessonite  Sp.  gr.  =  3.4-3.6 

Mg3Al2(Si04)3  Pyrope  =3.7-3.8 

MnaAl2(Si04)3  S pes sar tile  =4.1-4.3 

Fe3Al2(Si04)3     Almandite  =4.1-4.3 

Ca3Fe2(SiC>4)3    Andradite  or  Mdanite  =3.8-4.1 

Ca3Cr2(Si04)3    Uvarovite  =3-4 

The  following  table  contains  the  calculated  percentage  composition 
of  the  several  pure  garnet  molecules  and  the  records  of  analyses  of  some 
typical  varieties  of  the  mineral: 


It 

II* 

lib 

HI* 

IIIb 
IVa 

IVb 

Va 

vb 

Vo 
Via 
VIb 


SiO,  Al2Os 

40.01  22.69 

42.01  17.76 

44.78  25.40  .. 

40.92  22.45  5 
36.30  20.75  •• 
36.34  12.63  4 
36.15  20.51  43 
37.61  22.70  33 

35-45  3i 

35 .  09  tr.  29 

26.36  22 

38. 23  

36.93  5-68  1 


FcaOi     CnO,     FeO 


46 

•  • 

57 

34 

83 

49 

15 
00 

•  • 

96 


•   •  •  • 


•   •  •  • 


506 


8. 11 


MgO 

CaO 

•     •      •     a      a 

3730 

•13 

35  01 

29.82 

•    •  •  •    • 

17.85 

504 

.47     1.49 


2.49 


29.27 

21.84 


3.01 

1.44 

•    a    •     • 

3306 

•  24 

3280 

1.25 

30.72 

•    •    •     • 

29.27 

i-54 

3163 

MnO     TiO, 


20 

•   • 

46 

42.95 
44.20 

•    •  •  •   • 

12 


36 


tr. 


■     •     •    a     a 

•  •     •    •     • 

•  •     a     •     • 
a     •     a     a     a 

21.56 

a      a     a     a      a 


Total 
100.00 
100.17 
100.00 
100.39 
100.00 

99.70 
100.00 
100.31 
100.00 
100.48 
101.89 
100.00 

9958 


la.  Theoretical  composition  of  the  grossularite  molecule. 

lb.  Green  and  red  grossularite  from  the  limestone  at  Santa  Clara,  Cal. 

Ila-  Theoretical  composition  of  the  pure  pyrope  molecule. 

lib.  Pyrope  from  a  peridotite  in  Elliot  Co.,  Ky.    Also,  HjO  ■»  .10. 
Ilia.  Theoretical  composition  of  spessartite. 
IIIb.  Spessartite  from  Amelia  Court  House,  Va. 
IVa.  Theoretical  composition  of  almandite. 
IVb.  Almandite  from  Salida,  Colo. 

Va.  Theoretical  composition  of  andradite. 

Vb.  Andradite  from  East  Rock,  New  Haven,  Conn.    Also,  HjO  **  .35. 

Vc.  Schorlomite  from  Magnet  Cove,  Ark. 
Via-  Theoretical  composition  of  uvarovite. 
VIb-  Uvarovite  from  Bissersk,  Urals. 

The  crystals  of  garnet  are  usually  simple  combinations  of  00  O(no) 
(Fig.  166);  202(211)  and  often  3OK321)  (Figs.  167  and  168),  although 
all  the  other  holohedrons  are  also  occasionally  met  with.  Their  cleavage 
which  is  indistinct  is  parallel  to  00  O(uo). 


310  DESCRIPTIVE  MINERALOGY 

When  examined  in  polarized  light  many  garnets,  especially  those 
occurring  in  metamorphic  rocks,  are  doubly  refracting  and,  therefore, 
have  not  the  molecular  structure  belonging  to  isometric  crystals.    This 


Fig.  166.— Garnet  Crystal.    (Natural  size.)    Form:    °oO  (no). 


Ficj.  167.  Fie.  168. 

Fie.  167.— Garnet  Crystals  with  «0,  no  (d)  and  2O2,  211  (n). 
Fig.  168.— Garnet  Crystal  with  d  and  «  as  in  Fig.  16;.     Also  «  O2,  210  (c)  and  30|, 

131  w. 

phenomenon  has  been  explained  as  due  to  several  causes,  the  most  rea- 
sonable explanation  ascribing  it  to  strains  produced  in  the  crystals  upon 
cooling. 


ANHYDROUS  ORTHOSILICATES  311 

The  garnets  vary  in  color  according  to  their  composition,  the  com- 
monest color  being  reddish  brown.  Their  luster  is  vitreous,  their 
streak  white,  hardness  6-7.5,  and  density  3.4-4.3.  They  are  transparent 
or  translucent.  Most  varieties  are  easily  fusible  to  a  light  brown  or 
black  glass,  which  in  the  case  of  the  varieties  rich  in  iron  is  magnetic. 
Uvarovite,  however,  is  almost  infusible.  Some  garnets  are  unattacked 
by  acids;  others  are  partially  decomposed. 

Garnets,  when  in  crystals,  are  easily  distinguished  from  other  sim- 
ilarly crystallizing  substances  by  their  color  and  hardjiess.  Massive 
garnet  may  resemble  resuzianilct  sphene,  zircon  or  tourmaline.  It  is 
distinguished  from  zircon  by  its  easier  fusibility  and  from  vesuvianite 
by  its  more  difficult  fusibility;  from  tourmaline  by  its  higher  specific 
gravity,  and  from  sphene  by  the  reaction  from  titanium. 

Under  the  influence  of  the  air  and  moisture  garnets  may  be  partially 
or  entirely  changed  to  epidote,  muscovite,  chlorite,  serpentine,  and  oc- 
casionally to  other  substances. 

Grossularite,  Essonite,  Hessonite,  or  Cinnamon  Garnet  occurs 
principally  in  crystalline  schists  and  in  metamorphosed  limestones, 
where  it  is  associated  with  other  calcium  silicates.  It  is  found  also 
in  quartz  veins.  The  mineral  is  white,  bright  yellow,  cinnamon-brown 
or  some  pale  shade  of  green  or  red.  The  lighter-colored  varieties  are 
transparent  or  nearly  so.  Those  that  are  colored  are  used  as  gems. 
Much  of  the  hyacinth  of  the  jewelers  is  a  red  grossularite  (see  p.  317). 
Its  hardness  is  about  7  and  its  density  3.4-3.6.  It  is  fairly  easily 
fusible  before  the  blowpipe.  The  refractive  index  of  colorless  vari- 
eties for  yellow  light  is,  »=  1.7438. 

Good  crystals  of  grossularite  occur  at  Phippsburg,  Raymond  and 
Rumford,  in  Maine,  and  at  many  other  places  both  in  this  country  and 
abroad.    Bright  yellow  varieties  are  reported  from  Canyon  City,  Colo. 

Pyrope  is  deep  red,  sometimes  nearly  black.  Its  hardness  is  a  little 
greater  than  7  and  its  density  3.7.  Its  refractive  index  for  yellow  light 
is  between  1.74 12  and  1.7504.  The  pure  magnesium  garnet  is  unknown. 
All  pyropes  contain  admixtures  of  iron  and  calcium  molecules.  Many 
pyropes  are  transparent.  Those  with  a  dark  red  color  are  used  as  gems. 
They  occur  principally  in  basic  igneous  rocks. 

The  principal  occurrence  of  the  gem  variety  in  this  country  is  in 
Utah,  near  the  Arizona  line,  .about  100  miles  west  of  Ganado,  Ariz., 
where  it  is  found  lying  loose  in  wind-blown  sand. 

Rhodolite  is  a  pale  rose-red  or  purple  variety  from  Macon  Co.,  N.  C. 
It  consists  of  two  parts  pyrope  and  one  of  almandite, 


312  DESCRIPTIVE  MINERALOGY 

Spessartite  is  hyacinth  or  brownish  red,  with  occasionally  a  tinge 
of  violet.  The  purest  varieties  are  yellow,  but  since  there  is  nearly 
always  an  admixture  of  one  of  the  iron  molecules,  the  more  usual  color 
is  reddish  brown.  The  mineral  is  usually  transparent.  Its  hardness  is 
7  or  a  little  greater,  and  its  density  3.77-4.27.  Its  refractive  index  for 
yellow  light  is  1.8105.  In  the  blowpipe  flame  it  fuses  fairly  easily  to  a 
black,  nonmagnetic  mass,  and  with  borax  gives  an  amethyst  bead.  It 
is  found  in  acid  igneous  rocks  and  in  various  schists. 

Its  best  known  occurrences  in  the  United  States  are  in  granite,  at 
Haddam,  Conn.,  in  pegmatite,  at  Amelia  Court  House,  Va.,  and  in 
the  lithophyse  of  rhyolites,  near  Nathrop,  in  Colorado. 

Almandite  is  deep  red,  brownish  red  or  black.  It  is  one  of  the  com- 
monest of  all  garnets.  It  furnishes  nearly  all  the  material  manufactured 
into  abrasives.  Transparent  varieties  are  also  used  as  gems.  The  min- 
eral has  a  hardness  of  7  and  over.  Its  density  is  4.1-4.3,  and  its  refrac- 
tive index,  n,  for  yellow  light,  is  about  1.8100.  It  is  slightly  decom- 
posed by  HC1.  Before  the  blowpipe  it  fuses  to  a  dark  gray  or  black 
magnetic  mass.  It  is  found  in  granites  and  andesites,  and  also  in  various 
gneisses  and  schists  and  in  ore  veins. 

Its  best  known  occurrences  in  North  America  are  at  Yonkers  and 
at  various  points  in  the  Adirondacks,  N.  Y.,  at  Avondale,  Pa.,  and  on  the 
Stickeen  River,  in  Alaska. 

Andradite,  or  melanite,  is  black,  brown,  brownish  red,  green,  brown- 
ish yellow  or  topaz-yellow.  The  purest  varieties  are  topaz-yellow  or 
light  green  and  transparent.  The  former  constitute  the  gem  iopazolite 
and  the  latter,  demantoid.  The  black  variety,  melanite,  nearly  always 
contains  titanium.  It  occurs  in  alkaline  igneous  rocks,  in  serpentine, 
in  crystalline  schists  and  in  iron  ores.  The  most  titaniferous  varieties 
are  known  as  schorlomite.  The  hardness  of  andradite  is  about  7  and  its 
density  between  3.3  and  4.1.  n  for  yellow  light  =  1.8566.  It  is  fusible 
before  the  blowpipe  to  a  black  magnetic  mass. 

The  mineral  is  very  widely  spread.  It  occurs  at  Franklin,  N.  J.,  in 
metamorphosed  limestone;  near  Franconia,  N.  H.,  in  quartz  veins,  and 
at  many  other  places.  A  black  titaniferous  variety  occurs  in  a  meta- 
morphosed limestone  in  southwestern  California  and  near  Magnet  Cove, 
in  Arkansas.  The  variety  found  at  Magnet  Cove  is  schorlomite.  It  is  a 
black  glassy  mineral  associated  with  brookite  (TiCfe),  nepheline  (p.  314), 
and  thomsonite  (p.  455). 

Common  garnet  is  a  mixture  of  the  grossularite,  almandite  and 


ANHYDROUS  ORTHOSILICATES  313 

andradite  molecules.     It  occurs  in  many  metamorphosed  igneous  rocks 
and  in  some  slates. 

Uvarovite  is  emerald-green.  It  is  rare,  occurring  only  with  chromite 
in  serpentine  at  Bissersk  and  Kyschtim  in  the  Urals  and  in  the  chromite 
mines  at  Texas,  Penn.,  and  New  Idria,  Cal.  Its  hardness  is  about  7 
and  density  3.42.  Its  refractive  index  for  yellow  light  is  1.8384.  It  is 
infusible  before  the  blowpipe  but  dissolves  in  borax,  producing  a  green 
bead. 

Syntheses. — Garnet  crystals  have  been  produced  by  fusing  9  parts  of 
nepheline  and  1  part  of  augite  (p.  374).  The  fusion  results  in  a 
crystalline  mass  of  nepheline,  in  which  spinel  and  melanite  crystals  Are 
embedded. 

Occurrence. — The  members  of  the  garnet  group  are  widely  spread  in 
nature.  They  occur  in  schists,  slates  and  other  regionally  metamor- 
phosed rocks,  in  granite,  rhyolite  and  other  igneous  rocks,  and  as  con- 
tact products  in  limestones.  They  are  found  also  in  quartz  veins,  in 
pegmatite,  and  associated  with  other  silicates  in  ore  veins.  In  some 
instances  they  separated  from  a  cooling  magma,  in  others  they  are  the 
products  of  pneumatolitic  process,  and  in  others  they  are  the  results  of 
contact  and  dynamic  metamorphism. 

Uses  and  Production. — The  varieties  that  are  transparent  are  used 
as  gems.  Other  varieties  are  crushed  and  employed  as  abrasives.  The 
value  of  the  gem  material  produced  in  the  United  States  in  191 2  was 
$860.  The  production  for  abrasive  purposes  was  4,182  short  tons,  val- 
ued at  $137,800.  All  of  this  was  produced  in  the  mountain  regions  of 
New  York,  New  Hampshire  and  North  Carolina.  The  rock  is  crushed 
and  the  garnet  separated  by  hand  picking,  screening,  or  by  jigging. 
The  crushed  material  is  used  largely  in  the  manufacture  of  garnet  paper. 

NEPHELINE  GROUP 

The  nepheline  group  of  minerals  includes  three  closely  related  com- 
pounds, of  which  nepheline  is  the  most  common.  They  are  all  alumino- 
silicates  of  the  alkalies.  Nepheline  appears  to  be  a  solution  of  Si02, 
or  of  albite,  in  isomorphous  mixtures  of  the  orthosilicates,  NaAlSi04 
and  KAlSi04  in  the  proportion  of  8  molecules  of  the  silicates  to  one  of 
Si02,  thus: 

8(Na-  K)AlSi04+Si02=  (Na-  K)0((Na-  K)AlSi03)2Al6(Si04)7 

The  other  two  members  of  the  group  are  eucryptite  (LiAlSi04)  and 
kaliophilite  (KAlSi04). 


314 


DESCRIPTIVE  MINERALOGY 


The  members  of  the  group  crystallize  in  the  hexagonal  system  and 
are  apparently  holohedral,  but  nepheline  is  hemihedral  and  hemi- 
morphic  (hexagonal  pyramidal  class).  At  temperatures  above  1,248° 
the  nepheline  molecule  crystallizes  also  in  the  triclinic  system  as  car- 
negieUc  (see  p.  418). 


Nepheline  ((Na-K)8Al8Si©034) 

Although  approximately  a  potash-soda  silicate,  nearly  all  specimens 
of  nepheline  contain  more  or  less  CaO  and  nearly  all  contain  small 
quantities  of  water.  All  contain  an  excess  of  SiC>2.  To  avoid  the 
necessity  of  assuming  the  existence  of  this  SiC>2  in  solution  with 
(Na*  K)AlSiC>4,  it  has  been  suggested  that  the  variable  composition  of 
the  mineral  may  be  explained  by  regarding  it  as  a  solid  solution  of 
NaAlSiaOs  and  CaAkSi^s  (best  known  in  their  triclinic  forms  as 
albite  and  anorthite)  in  an  isomorphous  mixture  of  the  two  molecules, 
NaAISiCU  and  KAlSiO*.  The  average  of  five  analyses  of  crystals  from 
Monte  Somma,  Italy,  is  shown  in  I,  and  the  composition  of  a  mass  of 
the  mineral  from  Litchfield,  Maine,  in  II. 


Si02       AI2O3 

I.44.08      33.28 

II.  43 .74      34  •  48 


CaO      MgO   Na20        K20        H20 

1.57       .19      16.00        4.76         .15 
tr  tr       16.62        4.55         .86 


Total 
100.03 
100.25 


m 


m 


m 


When  found  in  crystals,  the  mineral  is  apparently  holohedral  in  form 
with  an  axial  ratio  1  :  .8389.    The  crystals  are  nearly  always  short 

columnar  in  habit  and  usually  consist  of  very 
/<^  c  ^>\  simple  combinations.  The  most  prominent 
NX     p     A^-il         forms  are  00  P(ioTo),    ooP2(ii2o),  oP(oooi), 

2P(202l),     P(ioll),     £P(l0l2)     and     2P2(lI2l) 

(Fig.  169).     Their  cleavage  is  imperfect  parallel 
to  00  P(  10T0)  and  oP  (0001 ) . 

Nepheline  is  glassy,  white  or  gray  and  trans- 
parent, when  occurring  as  implanted  crystals. 

xt    1  i-     ^        The  translucent  varietv  with    a  glassy  luster 
Fig.  169 —Nepheline  Crys-  .  .   -  *      f 

tal  with  oP  0001  U)'  ^at  occurs  m  rocks  is  known  as  eleohte.  This 
00  p,  10T0  (w);  P,  10T1  variety  may  be  gray,  pink,  brown,  yellowish  or 
(p)  and  00  P2, 1 1 20  (a),     greenish.     The  streak  is  always  white.     The 

fracture  of  both  forms  is  conchoidal  or  uneven; 
hardness,  5-6  and  density,  2.6.     For  yellow  light,  co=  1.5424,  6=1.5375. 


ANHYDROUS  ORTHOSILICATES  315 

Before  the  blowpipe  nepheline  melts  to  a  white  or  colorless  blebby 
glass.  At  1,248°  it  passes  over  into  carnegieite  which  melts  at  1,526°. 
It  dissolves  in  hydrochloric  acid. with  the  production  of  gelatinous 
silica.    Its  powder  before  and  after  roasting  reacts  alkaline. 

The  mineral  is  distinguished  from  other  silicates  by  its  crystalliza- 
tion, gelatinization  with  acids,  and  hardness.  The  massive  varieties 
are  often  distinguishable  by  their  greasy  luster. 

Nepheline  alters  to  various  hydrated  compounds,  especially  to  the 
zeolites  (p.  445),  and  to  gibbsite,  muscovite,  cancrinite  and  sodalite. 

Syntheses. — Nepheline  has  been  prepared  by  fusing  together  AI2O3, 
Si02  and  Na2CC>3,  and  by  the  treatment  of  muscovite  by  potassium 
hydroxide. 

Occurrence. — The  mineral  occurs  principally  as  an  original  constit- 
uent of  many  igneous  rocks,  both  plutonic  and  volcanic,  and  also  as 
crystals  on  walls  of  cavities  in  them. 

Localities. — Crystals  occur  near  Eberbach,  in  Baden;  in  the  inclu- 
sions within  volcanic  rocks  at  Lake  Laach,  in  Rhenish  Prussia;  in  the 
older  lavas  of  Monte  Somma,  Naples,  Italy;  at  Capo  de  Bove,  near 
Rome;  in  southern  Norway;  and  at  various  other  points  in  southern 
Europe.  Massive  forms  are  found  in  coarse-jjrained  rocks  near  Litch- 
field, Maine;  Red  Hill,  N.  H.;  Magnet  Cove,  Ark. ;  in  the  Crazy  Mts., 
Mont.,  and  at  other  places. 


Cancrinite  (He^CaMNaCOa^AlsCSiO^) 

Cancrinite  is  extremely  complex  in  composition.  It  is  nearly  allied 
to  nepheline  but  contains  a  notable  quantity  of  CO2.  It  corresponds 
approximately  to  an  hydrated  admixture  of  Na2CC>3  and  3NaAlSi04, 
in  which  some  of  the  Na  is  replaced  by  K  and  Ca.  Specimens  from 
Barkevik  (I)  in  Norway,  and  from  Litchfield  (II),  in  Maine,  yield  the 
following  analyses: 

Si02      AI2O3    Fe203      CaO    Na20    K20     C02     H20     Total 
I.  37.01       26.42       ....       7.19     18.36     ....     7.27    3.12      99.37 
II.  36.29      30.12        tr.        4.27     19.56       .18    6.96     2.98     100.36 

Cancrinite  is  hexagonal  (dihexagonal  bipyramidal  class). 

Crystals  are  rare,  and  those  that  do  exist  are  very  simple,  prismatic 
forms  bounded  by  ooP(ioTo),  ooP2(ii2~o),  oP(oooi)  and  P(ioTi). 
Their  axial  ratio  is  1  :  .4410. 


316  DESCRIPTIVE  MINERALOGY 

The  mineral  is  usually  found  without  crystal  planes.  It  is  colorless, 
white  or  some  light  shade,  such  as  rose,  bluish  gray  or  yellow.  Its 
streak  is  white,  its  luster  glassy,  greasy  or  pearly  and  it  is  translucent. 
Its  cleavage  is  perfect  parallel  to  ooP(ioTo)  and  less  perfect  parallel 
to  oo  P2  11 20).  Its  break  is  uneven;  hardness  5  and  density  2.45. 
For  red  light:  w=  1.5244,  €=1.4955. 

Before  the  blowpipe  the  mineral  loses  its  color,  swells  and  fuses  to  a 
colorless  blebby  glass.  In  the  closed  glass  tube  it  loses  CO2  and  water, 
and  becomes  opaque.  After  roasting  it  is  easily  attacked  by  weak 
acids  with  effervescence  and  the  production  of  gelatinous  silica.  When 
boiled  with  water  Na2CC>3  is  extracted  in  sufficient  quantity  to  give  an 
alkaline  reaction. 

Cancrinite  is  easily  distinguished  by  its  effervescence  with  acids  and 
the  production  of  gelatinous  silica. 

Synthesis. — Small  colorless,  hexagonal  crystals  with  a  composition 
corresponding  to  that  of  cancrinite,  have  been  made  by  treating  mus- 
covite  with  a  solution  of  NaOH  and  NaoCCb  at  5000. 

Occurrence. — The  mineral  occurs  principally  as  an  associate  of  neph- 
eline  in  certain  coarse-grained  igneous  rocks.  In  some  cases  it  appears 
to  be  an  original  rock  constituent  and  in  others  an  alteration  product  of 
nepheline.  It  sometimes  alters  to  natrolite  (see  p.  454),  forming  pseu- 
domorphs. 

Localities. — Cancrinite  is  found  in  rocks  at  Ditro,  Hungary;  at 
Barkevik  and  other  localities  in  southern  Norway,  where  it  occurs  in 
pegmatite  dikes;  in  the  parish  of  Kuolajarvi,  in  Finland,  and  in  nepheline 
syenite  at  Litchfield  in  Maine. 

ZIRCON  GROUP 

The  orthosilicates  of  zirconium,  zircon,  and  of  thorium,  thorite,  con- 
stitute a  group,  the  members  of  which  possess  forms  that  are  almost 
identical  with  those  of  rutile,  cassiterite  and  xenotime.  Indeed,  parallel 
growths  of  zircon  and  xenotime  are  not  uncommon.  Formerly  zircon 
was  grouped  with  the  two  oxides. 

Zircon  and  thorite  are  tetragonal  (ditetragonal  bipyramidal  class), 
with  approximately  the  same  axial  ratios  and  the  same  pyramidal  angles. 
The  two  minerals  are  completely  isomorphous. 

Zircon        ZrSi04     a  :  £=.6301      in  A  1^1  =  56°  37', 
Thorite      ThSi04  =.6402  =56°  40'. 

Zircon  is  fairly  common.     Thorite  is  rare. 


ANHYDROUS  ORTHOSILICATES 


317 


Zircon  (ZrSi04) 

Zircon,  like  rutile,  is  a  fairly  common  compound  of  a  comparatively 
rare  metal.  It  is  practically  the  only  ore  of  the  metal  zirconium.  It  is 
found  mainly  in  crystals  and  as  gravel. 

Although  some  specimens  of  zircon  contain  a  large  number  of  ele- 
ments, others  consist  only  of  zirconium,  silicon  and  oxygen  in  propor- 
tions that  correspond  to  the  formula  ZrSiCU,  which  demands  67.2  per 
cent  ZrO  and  32.8  per  cent  Si02. 

Its  axial  ratio  is  a  :  f=i  :  .6391.  Its  crystals  are  usually  simple 
combinations  of  00  P(no)  and  P(m),  with  the  addition  of  00  P  00  (100) 


m 


m 


Fig.  170. 


Fig.  171. 


Fig.    170. — Zircon   Crystals  with  00  P,   no  («);    00 Poo,  100   (a);   3P,  331    (*), 

P,  in  (p)  and  3P3,  311  (x). 
Fig.  171. — Zircon  Twinned  about  P  00  (101).    *=2P  (221). 


and  often  3P3(3ii)  (Fig.  170).  Elbow  twins,  like  those  of  rutile  and 
cassiterite,  are  known  (Fig.  171). 

The  cleavage  of  zircon  is  very  indistinct.  Its  fracture  is  conchoidal. 
Its  hardness  is  7.5  and  density  about  4.7.  The  mineral  varies  in  tint 
from  colorless,  through  yellowish  brown  to  reddish  brown.  Its  streak 
is  uncolored  and  luster  adamantine.  Most  varieties  are  opaque,  but 
transparent  varieties  are  not  uncommon.  The  orange,  brown  and  red- 
dish transparent  kinds  constitute  the  gem  known  as  hyacinth.  The 
refractive  indices  for  yellow  light  are:  «=  1.9302,  c=  1.9832. 

Zircon  is  infusible,  though  colored  varieties  often  lose  their  color 
when  strongly  heated.  In  the  borax  and  other  beads  the  mineral  gives 
no  preceptible  reactions.  In  fine  powder  it  is  decomposed  by  concen- 
trated sulphuric  acid.  When  fused  with  sodium  carbonate  on  platinum 
it  is  likewise  decomposed,  and  the  solution  formed  by  dissolving  the 
fused  mixture  in  dilute  hydrochloric  acid  turns  turmeric  paper  orange. 
This  is  a  characteristic  test  for  the  zirconium  salts. 


318  DESCRIPTIVE  MINERALOGY 

The  mineral  is  easily  recognized  by  its  hardness,  its  resistance  toward 
reagents  and  its  crystallization. 

Syntheses. — Small  crystals  of  zircon  are  obtained  by  heating  for  sev- 
eral hours  in  a  steam-tight  platinum  crucible  a  mixture  of  gelatinous 
silica  and  gelatinous  zirconium  hydroxide.  Crystals  have  also  been 
made  by  heating  for  a  month  a  mixture  of  ZnCfe  and  S1O2  with  6  times 
their  weight  of  lithium  bimolybdate. 

Occurrence  and  Origin. — Zircon  is  widely  spread  in  tiny  crystals  as  a 
primary  constituent  in  many  rocks,  and  in  large  crystals  in  a  few,  notably 
in  limestone  and  a  granite-like  rock  known  as  nepheline  syenite.  In 
limestone  it  is  a  product  of  contact  action.  It  occurs  also  in  sands, 
more  particularly  in  those  of  gold  regions,  and  abundantly  in  a  sand- 
stone near  Ashland,  Va. 

Localities. — The  principal  occurrences  of  the  mineral  are  Ceylon,  the 
home  of  the  gem  hyacinth;  the  gold  sands  of  Australia;  Arendal, 
Hakedal  and  other  places  in  Norway;  Litchfield  and  other  points  in 
Maine;  Diana,  in  Lewis  Co.,  and  a  large  number  of  other  places  in  New 
York;  at  Reading,  Penn.;  Henderson  and  other  Counties,  in  North 
Carolina  and  Templeton,  Ottawa  Co.,  Quebec. 

Uses. — Zircon  is  the  principal  source  of  the  zirconium  oxide  employed 
in  the  manufacture  of  gauze  used  in  incandescent  gas  lights  and  in  the 
manufacture  of  cylinders  for  use  in  procuring  a  light  from  the  oxyhydro- 
gen  jet.  The  mineral  has  been  mined  for  these  purposes  in  Henderson 
Co.,  North  Carolina. 

Transparent  orange-colored  zircons  are  sometimes  used  as  gems 
since  they  possess  a  high  index  of  refraction  and  consequently  have 
a  great  deal  of  "  fire."  These  are  the  true  .hyacinth.  The  mineral 
often  called  by  this  name  among  the  jewelers  is  a  yellowish  brown 
garnet. 

Production. — A  small  quantity  of  zircon  is  usually  obtained  from 
Henderson  Co.,  N.  C,  but  it  rarely  amounts  to  more  than  a  few  hundred 
pounds.  The  mineral  occurs  in  a  pegmatite  and  the  soil  overlying  its 
outcrop.  It  is  obtained  by  crushing  the  rock  and  hand  picking.  Usually 
there  is  a  little  also  separated  from  the  sands  in  North  Carolina  and 
South  Carolina  that  are  washed  for  monazite.  A  pegmatite  dike,  rich 
in  zircon,  is  also  being  prospected  in  the  Wichita  Mountains,  Okla.,  but 
no  mining  has  yet  been  attempted. 


ANHYDROUS  ORTHOSILICATES  319 

Thorite  (ThSi04) 

Thorite  occurs  in  simple  crystals  bounded  by  oo  P(no)  and  P(m) 
(Fig.  172),  and  in  masses.  The  mineral  is  always 
more  or  less  hydrated,  but  this  is  believed  to  be 
due  to  partial  weathering.  It  is  black  or  orange- 
yellow  (orangeite),  has  a  hardness  of  5  and  a  specific 
gravity  of  4.5-5  for  black  varieties  and  5.2-5.4  for 
orange  varieties.  Its  streak  is  brown  or  light  orange. 
Hydrated  specimens  are  soluble  in  hydrochloric  acid 
with  the  production  of  gelatinous  silica.  The  min-  Fig.  172.— Thorite 
eral  occurs  as  a  constituent  of  the  igneous  rock,  Crystal  with  *>  p, 
augite-syenite,  at  several  points  in  the  neighborhood  IIQ  '*'  an  ' 
of  the  Langesundf jord,  Norway, 

BASIC  ORTHOSILICATES 

ANDALUSITE  GROUP 

Three  compounds  with  the  empirical  formula  AkSiOs  exist  as  min- 
erals, kyanite,  or  disthene,  andalusite  and  sUlimanile.  The  first  named  is 
less  stable  with  reference  to  chemical  agents  than  the  other  two,  but  at 
high  temperatures  both  kyanite  and  andalusite  are  transformed  into 
sillimanite.  Kyanite  is  regarded  as  a  metasilicate  (A10)2SiC>3.  The 
other  two  are  thought  to  be  orthosilicates  (Al(AlO)SiC>4).  The  latter 
are  orthorhombic  and  both  possess  nearly  equal  prismatic  angles. 
They  differ  markedly,  however,  in  their  optical  and  other  physical 
properties  and,  therefore,  are  different  substances.  Kyanite  is  triclinic. 
For  this  reason  and  because  of  its  different  composition  it  is  not  re- 
garded as  a  member  of  the  andalusite  group.  A  fourth  mineral,  topaz, 
differs  from  andalusite  in  containing  fluorine.  Often  this  element  is 
present  in  sufficient  quantity  to  replace  all  of  the  oxygen  in  the  radical 
(AlO).  In  other  specimens  the  place  of  some  of  the  fluorine  is  taken 
by  hydroxyl  (OH).  The  general  formula  that  represents  these  varia- 
tions is  Al(Al(F-OH)2)Si04.  The  mineral  crystallizes  in  forms  that  are 
very  like  those  of  andalusite,  and  if  corresponding  pyramids  are  selected 
as  groundforms  their  axial  ratios  are  nearly  alike.  Unfortunately, 
however,  different  pyramids  have  been  accepted  as  groundforms,  and 
therefore  the  similarity  of  the  crystallization  of  the  two  minerals  has 
been  somewhat  obscured.  Danburite,  another  mineral  that  crystallizes 
in  the  orthorhombic  system  with,  a  habit  like  that  of  topaz  is  often  also 
placed  in  this  group,  although  it  is  a  borosilicate,  thus  CaB2(Si04)2. 


320 


DESCRIPTIVE  MINERALOGY 


If  4P2(24i)  be  taken  as  the  groundform  of  andalusite,  3P(33i)  as 
that  of  topaz  and  3P(33i)  as  that  of  danburite,  the  corresponding  axial 
ratios  would  be: 

Andalusite    a  :  b  :  c—  .5069  :  1  :  1 .4246 
Topaz  =.5281  :  1  :  1.4313 

Danburite  =  .  5445  :  1  :  1 .  4402 

These,  however,  are  not  the  accepted  ratios,  since  other  and  more  prom- 
inent pyramids  have  been  selected  as  the  groundforms. 

Andalusite  and  Sillimanite  (Al(A10)Si04) 

Andalusite  and  sillimanite  have  the  same  empirical  chemical  compo- 
sition and  crystallize  with  the  same  symmetry,  which  is  orthorhombic 
holohedral  (rhombic  bipyramidal  class),  but  they  have  different  physical 
properties  and  different  crystal  habits,  and  hence  are  regarded  as  dif- 
ferent minerals.    The  theoretical  composition  of  both  is: 

Si02= 37.02;  Al203  =  62.98.    Total=  100.00. 

Nearly  all  specimens  when  analyzed  show  the  presence  of  small 
quantities  of  Fe,  Mg,  and  Ca,  but  otherwise  they  correspond  very  closely 
to  the  theoretical  composition. 

Both  minerals  are  characteristic  of  metamorphosed  rocks,  but 
andalusite  occurs  principally  in  those  that  have  been  metamorphosed  by 

contact  with  igneous  intru- 
sives,  while  sillimanite  is 
especially  characteristic  of 
crystalline  schists  and,  in  gen- 
eral, of  rocks  that  were  dy- 
namically metamorphosed.  It 
also  occurs  with  olivine  as  in- 
clusions in  basalt  lavas.     Silli- 


m 


m 


_V 


Fig.  173.— Andalusite  Crystals  with  <»  p,  no  manite  is  more  stable  at  high 

jw»;^l(*  P»£ii_(j);  oops    100  temperatures  than  andalusite. 

/  x    r>-OI°    ,\    t*  2>  "/^v      J"  t>-'   When  in  contact  rocks  it  is 

120  (»);  Poo,  101  (r);  P,  111  (p)  and  2P2, 

121  (£).  found    nearer    the    intrusive 

than  andalusite. 
Andalusite. — The  accepted  axial  ratio  of  andalusite  is  .9861  :  1  :  .7024. 
Its  crystals  are  columnar  in  habit  and  are  usually  simple  combinations 

of  00  P  GO  (lOo),   00  P  06  (OIO),  OP(OOI),   00  P(lio),   00  P2(2I0),   00  P2(l20) 

P«  (101),  Poo  (on)  with  sometimes  P  (in)  and  2P2(i2i)  (Fig.  173). 
The  angle  no  A  110=89°  I2'- 


ANHYDROUS  ORTHOSILICATES  321 

The  mineral,  when  fresh,  is  greenish  or  reddish  and  transparent. 
Usually,  however,  it  is  more  or  less  altered,  and  is  opaque,  or,  at  most, 
translucent,  and  gray,  pink  of  violet.  Its  cleavage  is  good  parallel  to 
oo  P(no)  and  its  fracture  uneven.  Its  hardness  is  7  or  a  little  less  and 
its  density  3.1-3.2.  In  some  specimens  pleochroism  is  marked,  their 
colors  being  olive-green  for  the  ray  vibrating  parallel  to  a,  oil-green  for 
that  vibrating  parallel  to  b  and  dark  red  for  that  vibrating  parallel  to  c. 
For  yellow  light  the  indices  of  refraction  are:  0=1.6326,  18=1.6390, 
7=1.6440. 

Before  the  blowpipe  the  mineral  gradually  changes  to  sillimanite  and 
is  infusible.  When  moistened  with  cobalt  nitrate  and  roasted  it  becomes 
blue.    It  is  insoluble  in  acids. 

The  mineral  is  distinguished  by  its  nearly  square  cross-section,  its 
hardness,  its  infusibility,  and  the  reaction  for  Al,  and  by  its  manner  of 
occurrence  in  schists  and  metamorphosed  slates. 

Some  specimens  contain  as  inclusions  large  quantities  of  a  dark 
gray  or  black  material,  which  may  be  carbonaceous,  arranged  in 
such  a  way  as  to  give  a  cross-like  figure  in  cross-sections  of  crystals. 
Because  of  the  shape  of  the  figure  exhibited  by  these  crystals,  this 
variety  was  early  called  chiastolite,  and  was  valued  as  a  sacred 
charm.  1 

Andalusite  alters  readily  to  kaolin  (p.  404),  muscovite  (p.  355),  and 
sillimanite.    It  has  not  been  produced  artificially. 

Occurrence. — Andalusite  is  found  principally  in  clay  slates  and  schists 
that  have  been  metamorphosed  by  contact  with  igneous  masses,  and 
to  a  less  extent  in  gneisses. 

Localities. — Its  principal  occurrences  are  in  Andalusia,  Spain;  at 
Braunsdorf,  Saxony;  at  Gefrees,  in  the  Fichtelgebirge;  in  Minas 
Geraes,  Brazil,  and  in  the  United  States  at  Standish,  Maine;  Westford, 
Mass.,  and  Litchfield,  Conn.  Chiastolite  occurs  at  Lancaster  and 
Sterling,  Mass. 

Use. — The  only  use  to  which  andalusite  has  been  put  is  as  a  semi- 
precious stone,  and  for  this  purpose  only  the  chiastolite  variety  is  of  any 
value. 

Sillimanite,  or  fibrolite,  occurs  principally  in  acicular  or  fibrous 
aggregates,  on  the  individuals  of  which  only  the  prismatic  forms 
ooP(no)  and  00  P|(23o)  and  the  macropinacoid  00  Poo  (100)  can  be 
detected.  End  faces  are  not  sufficiently  developed  to  warrant  the 
determination  of  an  axial  ratio.  The  relative  values  of  the  a  and  b 
axes  are  .687  :  1.    The  angle  no  A  110=69°. 

While  most  of  the  fibers  correspond  in  composition  very  closely  to  the 


322  DESCRIPTIVE  MINERALOGY 

theoretical  value  demanded  by  the  formula  Al(A10)Si04,  many  contain 
small  quantities  of  Fe^,  MgO  and  H2O. 

The  mineral  is  yellowish  gray,  greenish  gray,  olive-green  or  brownish. 
It  has  a  glassy  or  greasy  luster  and  when  pure  is  transparent.  Most 
specimens,  however,  are  translucent,  and  many  of  the  colored  varieties 
show  a  pleochroism  in  brown  or  reddish  tints.  Its  cleavage  is  perfect 
parallel  to  00P60  (100).  Its  needles  have  an  uneven  fracture  trans- 
versely to  their  long  directions.  Their  streak  is  colorless,  hardness 
6-7  and  density  3.24.  The  indices  of  refraction  for  the  lighter  colored 
varieties  are:  a=  1.6603,  £=1.6612,  7=1.6818  for  yellow  light. 

Sillimanite  reacts  similarly  to  andalusite  toward  reagents  and  before 
the  blowpipe.  It  is  distinguished  from  other  minerals  by  its  habit  and 
manner  of  occurrence. 

This  mineral  is  much  more  resistant  to  weathering  than  is  andalusite. 
It  is,  however,  occasionally  found  altered  to  kaolin.  On  the  other  hand, 
it  is  known  also  in  pseudomorphs  after  corundum. 

Synthesis. — It  has  been  produced  by  cooling  fused  silicate  solutions 
rich  in  aluminium. 

Occurrence. — Sillimanite  is  very  widely  spread  in  schistose  rocks, 
especially  those  that  have  been  formed  from  sediments.  It  is  essentially 
a  product  of  dynamic  metamorphism,  but  is  formed  also  bv  contact 
metamorphism,  in  which  case  it  is  found  near  the  intrusive,  where  the 
temperature  was  high. 

Localities. — Its  principal  occurrences  in  North  America  are  in  quartz 
veins  cutting  gneisses  at  Chester,  Conn.,  at  many  points  in  Delaware 
Co.,  Penn.,  and  at  the  Culsagee  Mine,  Macon  Co.,  N.  C.  At  the  lAtter 
place  and  at  Media  in  Penn.,  a  fibrous  variety  occurs  in  such  large 

masses  as  to  constitute  a  schist — known  as  fibrolite  schist.       •    r-\ 

{  • 

Topaz  (Al(Al(F-0H)2)Si04) 

Topaz  is  a  common  constituent  of  many  ore  veins  and  is  often  present 
on  the  walls  of  cracks  and  cavities  in  volcanic  rocks.  It  occurs  massive 
and  also  in  distinct  and  handsome  crystals. 

The  mineral  has  a  varying  composition,  which  is  explained  in  part 
by  the  fact  that  it  is  a  mixture  of  the  two- molecules  Al(AlF2)Si04  and 
Al(Al(OH)2)SiOi.  The  theoretical  composition  of  the  fluorine  molecule 
is:  Si02=32.6,  Al203  =  55.4;  F=  20.7=  108.7;  deduct  (0=2F)8.7 
s=  100.00.    A  specimen  from  Florissant,  Colo.,  gave: 

Si02=33-I5;  Al2Oa  =  57.01;  F  =16.04  =106.20- 6.75(0  «=F)  =  99.45. 


ANHYDROUS  ORTHOSILICATES 


323 


Crystals  of  topaz  appear  to  be  orthorhombic  (rhombic  bipyramidal 
class),  but  the  fact  that  they  are  pyroelectric  and  that  they  frequently 
exhibit  optical  phenomena  that  are  not  in  accord  with  the  symmetry  of 
orthorhombic  holohedrons  suggests  that  they  may  possess  a  lower  grade 
of  symmetry.  On  the  assumption  that  the  mineral  crystallizes  with  the 
symmetry  of  orthorhombic  holohedronf  the  axial  ratio  of  fluorine  varie- 
ties is  .5281  :.i  :  -477r.1  With  the  increasing  presence  of  OH,  however, 
the  relative  length  of  a  increases  and  that  of  c  diminishes.  The  angle 
noAiTo=S5°  43'- 

The  crystals  are  usually  prismatic  in  habit  with  w>P(no)  and 
00  PJ(i2o)  predominating.    They  are  notable  for  the  number  of  forms 


f&>\ 


Fie.  174- 
FlG.174.— Topaz  Crystals  with  »P,iio{m);   °°  1 
4P » ,  041  00  and  »P« 
Fic.  175.— Topaz  Crystal  with  m,  I,  n  and  y  as 

*P«,043(A-)and2P« 


Fig.  i7S. 

r,i»(0;  P.iiiMi   iP»niM 

>.o  (6). 

1  Fig.  174.    Also  2p«,o2i  (J); 

2OI    (d). 


that  have  been  observed  on  them,  especially  in  the  prismatic  zone  and 
among  the  brachypyramids.  The  number  of  the  latter  that  have 
already  been  identified  is  about  45. 

The  three  types  of  crystals  that  are  most  common  are  shown  in 
Figs.  174,  175  and  176.  Their  most  prominent  forms  are  oop(no), 
■oPa(iao),  P»(on),  P(ni),  |P(233).  4P*(o4i),  « P3d3°)  and 
oP(ooi).  Often  planes  are  absent  from  one  end  of  the  vertical  axis, 
but  since  the  etch  figures  on  the  prismatic  planes  do  not  indicate  hemi- 
morphism,  the  absence  of  the  lacking  planes  is  explained  as  being  due  to 
unequal  growth.    The  planes  of  the  prismatic  zone  are  usually  striated. 

The  mineral  is  colorless,  honey  yellow,  yellowish  red,  rose  and  rarely 
bluish.    When  exposed  to  the  sunlight  the  colored  varieties  fade,  and 

inly  accepted  axial  ratio  is  a  :  b  :  t=  .5^85  :i  :  .0539,  the  form 


Then 


aP(aai)  being  taken  as  the  groundform. 


324 


DESCRIPTIVE  MINERALOGY 


m 


when  intensely  heated  some  honey-yellow  crystals  turn  rose-red.  Its 
cleavage  is  perfect  parallel  to  oP(ooi)  and  imperfect  parallel  to  P  06  (on) 
and  P  *  (101).  The  hardness  of  the  mineral  is  8  and  its  density  3. 5-3. 6. 
Its  refractive  indices  for  yellow  light  are:  a=  1.6072, 0=  1.6104, 7=  1.6 176 
for  a  variety  containing   very  little  OH,  and  a  =1.6294,  0=1.6308, 

7=1.6375  for  a  variety  rich  in 
hydroxyl.  The  indices  of  refraction 
being  high,  the  mineral  when  cut 
exhibits  much  brilliancy — a  feature 
which,  together  with  its  hardness, 
gives  it  much  of  its  value  as  a 
gem. 

Topaz    is   infusible    before  the 

blowpipe  and  is  insoluble  in  acids. 

Fig.  176.— Topaz  Crystal  with  m,  I,  y,    At  a  high   temperature  it  loses  its 

/",  d,  O  and  u  as  in  Figs.  174  and  175.    fluorine  as  silicon  and   aluminium 

Also  JP,  223  (0;   oP,  001  (c)  and   fluorides.      The   mineral   also   ex- 

9  4  hibits  pyroelectrical  properties,  but 

these    are    apparently    distributed    without    regularity    in    different 

crystals.    Many  crystals  contain  inclusions  of  fluids  containing  bubbles, 

and  sometimes  of  two  immiscible  fluids  the  nature  of  which  has  not  vet 

been  determined.     It  has  been  thought  that  the  principal  fluid  present 

is  liquid  carbon-dioxide  or  some  hydrocarbon. 

The  mineral  is  distinguished  from  yellow  quartz  by  its  crystalliza- 
tion, its  greater  hardness  and  its  easy  cleavage. 

Topaz  is  frequently  found  coated  with  a  micaceous  alteration  product 
which  may  be  steatite  (p.  401),  muscovite  (p.  355)  01  kaolin  (p.  404). 

Synthesis. — Crystals  have  been  made  by  the  action  of  hydrofluosilicic 
acid  (I^SiFe)  upon  a  mixture  of  silica  and  alumina  in  the  presence  of 
water  at  a  temperature  of  about  500  °. 

Occurrence. — The  mineral  occurs  principally  in  pegmatites,  espe- 
cially those  containing  cassiterite,  in  gneisses,  and  in  acid  volcanic  rocks. 
In  all  cases  it  is  probably  the  result  of  the  escape  of  fluorine-bearing 
gases  from  cooling  igneous  magmas. 

Localities. — Topaz  is  found  in  handsome  crystals  at  Schneckenstein 
in  Saxony,  in  a  breccia  made  up  of  fragments  of  a  tourmaline*quartz 
rock  cemented  by  topaz.  It  occurs  also  in  the  pegmatites  of  the  tin 
mines  in  Ehrenfriedersdorf,  Marienberg  and  other  places  in  Saxony, 
Bohemia,  England,  etc.;  on  the  walls  of  cavities  in  a  coarse  granite  in 
Jekaterinburg  and  the  Ilmengebirge,  Russia;  in  veins  of  kaolin  cutting 
a  talc  schist  in  Minas  Geraes  in  Brazil;  and  in  the  cassiterite-bearing 


ANHYDROUS  ORTHOSILICATES  325 

sands  at  San  Luis  Potosi,  Durango  and  other  points  in  Mexico.  In  the 
United  States  it  occurs  on  the  walls  of  cavities  in  acid  volcanic  rocks,  at 
Nathrop,  Colo.,  in  the  Thomas  Range,  Utah,  and  other  places.  It  occurs 
also  in  veins  with  muscovite,  fluorite,  diaspore  and  other  minerals  at 
Stoneham,  Maine,  and  Trumbull,  Conn. 

Uses  and  Production. — Topaz  is  used  as  a  gem.  About  36  lb.,  valued 
at  $2,675,  was  produced  in  the  United  States  in  ion.  In  the  following 
year  the  production  was  valued  at  only  $375. 

Danburite  (CaBz(Si04)z) 

Danburite,  which  is  a  comparatively  rare  mineral,  is  a  calcium 
borosilicate  with  the  following  theoretical  composition:  SK)2=48.84; 
Bz03  =  28.39  and  CaO=  22.77.  Usually,  however,  there  are  present  in  it 
small  quantities  of  AI2O3,  Fea03,  M^Oa  and  HzO.  Thus,  crystals  from 
Russell,  New  York,  contain: 

S1O2  BjOa        AI2O3,  etc.  HzO         CaO         Total 

49.70  25.80  1.02  .20  23.26        99.98 

The  mineral  crystallizes  in  the  orthorhombic  system  (rhombic  bipy- 
ramidal  class),  with  an  axial  ratio  .5445  :  1  :  .4801.    Its  crystals  are 
usually  prismatic  in  habit.    They  contain  a  great  number  of  forms,  of 
which  wPoo(ioo),  ooP66(oio),  «P2(i2o),  ooPJf^o),  and  <*>P(no) 
among  the  prisms,  2P4(i42),  2P2(i2i)  among 
the  pyramids  and  oP(ooi)  are  the  most  prom- 
inent   (Fig.    177).     The   angle    noAiio= 
57°  8'. 

When  fresh  and  pure  the  mineral  is  trans- 
parent, colorless  or  light  yellow,  but  when 
more  or  less  impure  is  pink,  honey-yellow  or 
dark  brown.    Its  streak  is  white,  and  luster 

vitreous.  Its  cleavage  is  imperfect  parallel  to  fic.  177.— Danburite  Crys- 
oP(ooi)  and  its  fracture  uneven  or  conchoidal.  tal  with  »  P,  no  (m); 
Its  hardness  is  about  7  and  density  2.95-3.02.  *>Pi,  j»  (/);  P»,  101 
Its  refractive  indices  for  yellow  light  are:  <<0;  *pM"  Wand4P«, 
«=r.63i7,  (3=1-6337,  7=1-6383-  °4'  W' 

Before  the  blowpipe  the  mineral  fuses  to  a  colorless  glass  and  colors 
the  8ame  green.  It  is  only  slightly  attacked  by  hydrochloric  acid,  but 
after  roasting  is  decomposed  with  the  formation  of  gelatinous  silica. 
It  phosphoresces  on  heating,  glowing  with  a  red  light. 

Origin. — Danburite  is  probably  always  a  product  of  pneumatolytic 


326  DESCRIPTIVE  MINERALOGY 

action,  as  it  is  found  in  quartz  and  pegmatite  veins  in  the  vicinity  of 
igneous  rocks  and  on  the  walls  of  hollows  within  them. 

Localities. — Its  principal  occurrences  in  this  country  are  at  Danbury, 
Conn.,  where  it  is  in  a  pegmatite,  and  at  Russell,  N.  Y.,  on  the  walls  of 
rocks  and  hollows  in  a  granitic  rock.  Its  principal  foreign  occurrence  is 
at  Piz  Valatscha,  in  Switzerland. 

EPIDOTE  GROUP  (Ca,R'"a(OH)(Si04),) 

The  epidote  group  comprises  six  substances,  of  which  two  are  di- 
morphs  with  thecomposition  Ca2Al3(OH)  (Si04)3  =  Ca2Al2(A10H)  (SiQi)3. 
One  of  these,  known  as  zoisitey  crystallizes  in  the  orthorhombic  system, 
and  the  other,  known  as  clinozoisite,  in  the  monoclinic  system.  The 
other  four  are  isomorphous  with  clinozoisite.  These  are  hancockUe,- 
epidote,  piedtnontite  and  allanite.  The  composition  and  comparative 
axial  ratios  of  the  four  commoner  isomorphs  are  as  follows  (assuming 
$P(Ti2)  as  the  groundform  of  clinozoisite): 

Clinozoisite  Ca2Al3(OH)(Si04)3  1-4457  •  i  •  18057 

Epidote  Ca2(AlFe)3(0H)(Si04)3  15807  :  1  :  1.8057,  $=64°  36' 
Piedmontite  Ca2(Al •  Mn)3(0H)(Si04)3  1.6100  :  1  :  1.8326,  £=64°  39' 
Allanite        Ca2(Al-Ce-Fe)3(OH)(Si04)3  1.5509  :  1  :  1.7691,  £=64°  59' 

Clinozoisite  is  rare,  though  its  molecule  occurs  abundantly  in  iso- 
morphous mixtures  with  the  corresponding  iron  molecule  in  epidote. 

Zoisite  (Ca2Al3(OH)(Si04)3) 

Zoisite  is  a  calcium,  aluminium  orthosilicate  containing  only  a  small 
quantity  of  the  corresponding  iron  molecule.  The  theoretical  composi- 
tion of  the  pure  Ca  molecule  is: 

SiO=39-52;   Al203  =  33i92;    CaO=  24.59;    H20=i.97.    Total  =  100.00. 

Colored  varieties  contain  a  little  iron  or  manganese.  Green  crystals  (I), 
from  Ducktown,  Tenn.,  and  red  crystals  (thulite)  (II),  from  Kleppan,  in 
Norway,  analyze  as  follows: 

Si02   AI2O3    Fe203  FeO    CaO   MgO  Mn203  Na20    H20    Total 

I.  39.61  32.89       .91     .71     24.50     .14     2.12     100.88 

II.  42.81  31.14     2.29     ...     18.73     ••■     1-^3     1&9       .64      99.13 

Zoisite  crystallizes  in  the  orthorhombic  system  (orthorhombic  bi- 
pyramidal  class),  with  the  axial  ratio  .6196  :  1  :  .3429.    Its  crystals  are 


ANHYDROUS  ORTHOSILICATES 


327 


m 


.-I 


^b 


usuaUy  simple  and  without  end  faces.  The  most  frequent  forms  are 
ooP(no),  ooP4(i4o),  oo  P  06(010).  P(iii),  2P  06  (021)  and 4P  06  (041) 
are  the  commonest  terminations  (Fig.  178).  The  crystals  are  all  pris- 
matic and  are  striated  longitudinally.  Their 
cleavage  is  perfect  parallel  to  00  P  06  (010). 
The  angle  noAiTo=63°  34'. 

The  mineral  is  ash-gray,  yellowish  gray, 
greenish  white,  green  or  red  in  color  and  has  a 
white  streak.  The  rose-red  variety,  contain- 
ing manganese,  is  known  as  thulite.  Very 
pure  fresh  zoisite  is  transparent,  but  the  ordi- 
nary forms  of  the  mineral  are  translucent. 
Its  luster  is  glassy,  except  on  the  cleavage 
surface,  where  it  is  sometimes  pearly.  Its 
fracture  is  uneven.  Its  hardness  is  6  and 
density  about  3.3.  In  specimens  from  Duck- 
town,  Term.,  a=  1.7002,  0=  1.7025,  7=1.7058 
for  yellow  light.  A  notable  fact  in  connection 
with  this  mineral  is  that  with  increase  of  the 
molecule  Ca2Fe3(OH)(Si04)3  in  the  mixture 
the  plane  of  its  optical  axes  tends  to  change 
from  oP(oio)  to  00  P  06  (001). 

Zoisite  fuses  to  a  clear  glass  before  the  blowpipe  and  gives  off  water, 
which  causes  a  bubbling  on  the  edges  of  the  heated  fragments.  It  is 
only  slightly  affected  by  acids,  but  after  heating  it  is  decomposed  by 
hydrochloric  acid  with  the  production  of  gelatinous  silica. 

Occurrence, — The  mineral  occurs  as  a  constituent  of  crystalline 
schists,  especially  those  rich  in  hornblende,  or  of  quartz  veins  traversing 
them.  It  is  also  a  component  of  the  alteration  product  known  as 
saussurite  which  results  from  the  decomposition  of  the  plagioclase 
(p.  418)  in  certain  basic,  augitic  rocks  known  as  gabbros.  It  is  thus  a 
product  of  metamorphism. 

Localities, — Good  crystals  of  zoisite  are  found  near  Pregratten  in 
Tyrol;  at  Kleppan  (thulite),  Parish  Souland,  Norway,  and  in  the  ore 
veins  at  the  copper  mines  of  Ducktown,  Tenn.,  where  it  is  associated  with 
chalcopyrite,  pyrite  and  quartz. 

Epidote  (Ca2(Al-Fe)3(OH)(Si04)3) 

Epidote,  or  pistazite,  differs  from  the  monoclinic  dimorph  of  zoisite 
(clinozoisite)  in  containing  an  admixture  of  the  corresponding  iron  sili- 
cate which  is  unknown  as  an  independent  mineral. 


Fig.  i  78. — Zoisite  Crystal 
with  00  P,  no  (m);  00  P  06  ; 
010  (6);  00  P4,  140  (/), 
2P00,  021  («)  and  P,  in 

W- 


328 


DESCRIPTIVE  MINERALOGY 


Since  it  consists  of  a  mixture  of  an  aluminium  and  an  iron  compound 
its  composition  necessarily  varies.  The  four  lines  of  figures  below  give 
the  calculated  composition  of  mixtures  containing  15  per  cent,  21  per 
cent,  30  per  cent  and  40  per  cent  of  the  iron  molecule. 

Percent  SiCk  AI2O3  Fe203  CaO  H20  Total 

15 38.60  28.80    6.65  24.02  1.93  100.00 

21 3823  26.76  9.32  23.78  1. 91  100.00 

30 37-67  23.71  13.31  23.43  1.88  100.00 

40 37  °4  20.32  17.75  23.04  1.85  100.00 

Most  specimens  contain  small  quantities  of  Mg,  Fe,  Mn,  Na  or  K. 

Epidote  is  isomorphous  with  clinozoisite,  crystallizing  in  the  mono- 


Fig.  179. — Epidote  Crystals  with  00  P  00  ,  100  (a);  oP,  001  (c);  P  00  ,  10 1  (r);  JP  00  , 

102  (1);  P,  11T  (»)  and  P  00  ,  on  (0). 


Fig.    180. — Epidote  Crystals  with   a,  c,  r,  f,  n  and  0  as  in  Fig.  179.    Also  00  P, 
no(m);   2P60,  2oT(/);    -P66,  101  (c);    —$P\,  231  (p)  and  |P2,  423  (/). 


clinic  system  (monoclinic  prismatic  class),  with  the  axial  ratio  1.5787  :  1 
:  1.8036.  0=64°  36'.  The  mineral  is  remarkable  for  its  handsome 
crystals,  many  of  which  are  extremely  rich  in  forms.  The  crystals  are 
usually  columnar  in  consequence  of  their  elongation  parallel  to  the  b 
axis.  The  most  prominent  forms  are  00  P  60  (100) ,  oP(ooi),  £P  66  (20T). 
Poo  (ioT),P(iiT),  00  P(no)  and  Pob  (on)  (Fig.  179  and  180).  In  addi- 
tion to  these,  over  300  other  forms  have  been  identified.  Twinning 
is  common,  with  00  Poo  (100)  the  twinning  plane.  The  angle  no  A 
iTo=  1090  56'. 

Epidote  is  yellowish  green,  pistachio  green,  dark  green,  brown  or, 
rarely,  red.  It  is  transparent  or  translucent  and  strongly  pleochroic. 
In  green  varieties  the  ray  vibrating  parallel  to  the  b  axis  is  brown,  that 
vibrating  nearly  parallel  to  c,  yellow,  and  that  vibrating  perpendicular  to 


ANHYDROUS  ORTHOSILICATES  329 

the  plane  of  these  two  is  green.  Its  luster  is  glassy  and  its  streak  gray. 
Its  cleavage  is  very  perfect  parallel  to  oP(ooi).  Its  hardness  is  6.5  and 
density  3.3  to  3.5.  The  refractive  indices  for  yellow  light  in  a  crystal 
from  Zillerthal  are:  0=1.7238,  13=1.7291,7=1.7343.  They  increase 
with  the  proportion  of  the  iron  molecule  present,  being  1.7336,  1.7593 
and  1. 7710  in  a  specimen  containing  27  per  cent  of  the  iron  epidote. 

The  varieties  that  have  been  given  distinct  names  are: 

Bucklandite,  a  greenish  black  variety  in  crystals  that  are  not  elon- 
gated; 

Withamite,  a  bright  red  variety  containing  a  little  MnO. 

Fragments  of  the  mineral  when  heated  before  the  blowpipe  yield 
water  and  fuse  to  a  dark  brown  or  black  mass  that  is  often  magnetic. 
With  increase  in  iron  fusion  becomes  more  easy.  Before  fusion  epidote 
is  practically  insoluble  in  acid.  After  heating  HC1  decomposes  it  with 
the  separation  of  gelatinous  silica. 

The  ordinary  forms  of  the  mineral  are  characterized  by  their  yellow- 
ish green  color,  ready  fusibility  and  crystallization. 

Occurrence  and  Origin. — Epidote  occurs  in  massive  veins  cutting  crys- 
talline schists  and  igneous  rocks,  as  isolated  crystals  and  druses  on  the 
walls  of  fissures  through  almost  any  rock  and  in  any  cavities  that  may 
be  in  them,  and  as  the  principal  constituent  of  the  rock  known  as  epi- 
dosite.  It  is  a  common  alteration  product  of  the  feldspars  (p.  408), 
pyroxenes  (p.  364),  garnet,  and  other  calcium  and  iron-bearing  minerals. 
Pseudomorphs  of  epidote  after  these  minerals  are  well  known.  The 
mineral  is  a  weathering  product,  but  is  more  commonly  a  product  of 
contact  and  regional  metamorphism. 

It  has  not  been  produced  artificially. 

Localities. — Epidote  crystals  are  so  widely  spread  that  only  a  few  of 
the  important  localities  in  which  they  have  been  found  can  be  mentioned 
here.  Particularly  fine  crystals  occur  in  the  Sulzbachthal,  Salzburg, 
Austria;  in  the  Zillerthal,  in  Tyrol;  near  Zermatt,  in  Switzerland;  in 
the  Alathal,  Traversella,  Italy;  at  Arendal,  Norway;  in  Japan,  at 
Prince  of  Wales  Island,  Alaska,  and  at  many  other  points  in  North 
America. 

Piedmontite  (Ca2(Al-Mn)3(OH)(Si04)3) 

Piedmontite  is  the  manganese  epidote,  differing  from  the  ordinary 
epidote  in  possessing  manganese  in  place  of  iron.  Usually,  however, 
the  iron  and  the  manganese  molecules  are  both  present.  Typical  analy- 
ses of  crystals  from  St.  Marcel,  in  Piedmont,  Italy  (I),  Otakisan,  Japan 
(II),  and  Pine  Mt.,  near  Monterey,  Md.  (Ill),  follow: 


330  DESCRIPTIVE  MINERALOGY 

S1O2     AI2Q3    M112O3  MnO  Fe2Q8  MgO   CaO    H2O     Total 
I.    35.68    18.93    14.27    3.22     1.34     ...     24.32    2.24     100.00 
II.    36.16     22.52      6.43*  9.33     .40    22.05   .3-2°     100.53* 

III.     47-37     i8-55       685     x-92     402     .25     15.82     2.08     100.05* 
* II.  contains  also  .44  per  cent  NatO.    The  MniO*  contained  also  MnO. 

III.  contains  also  2.03  per  cent  of  the  oxides  of  rare  earths,  .14  per  cent  PbO, 
.11  per  cent  CuO,  .23  per  cent  Na.O  and  .68  per  cent  KsO.  The  specimen  contained 
also  a  little  admixed  quartz,  which  was  determined  with  the  SiO*. 

The  axial  ratio  of  piedmontite  is  1.6100  :  1  :  1.8326.  0=64°  39'. 
Its  crystals  are  similar  in  habit  to  those  of  epidote,  but  they  are  .much 
simpler.  The  most  prominent  forms  are  00  P «»  (100),  oP(ooi),  P(Tn), 
$P  00(102),  00  Poo  (010)  and  ooP(no).  Twins  are  fairly  common, 
with  00  P  60  (100)  the  twinning  plane. 

The  mineral  is  rose-red,  brownish  red  or  reddish  black.  It  is  trans- 
parent or  translucent  and  strongly  pleochroic  in  yellow  and  red  tints 
and  has  a  glassy  luster  and  pink  streak.  It  is  brittle,  and  has  a  good 
cleavage  parallel  to  oP(ooi).  Its  hardness  is  6  and  density  3.40.  Its 
refractive  indices  are  the  same  as  those  of  epidote. 

Before  the  blowpipe  piedmontite  melts  to  a  blebby  black  glass  and 
gives  the  manganese  reaction  in  the  borax  bead.  It  is  unattacked  by 
acids  until  after  heating,  when  it  decomposes  in  HC1  with  the  separation 
of  gelatinous  silica. 

It  is  characterized  by  its  color  and  hardness  and  by  its  manganese 
reaction. 

Occurrence  and  Origin. — Piedmontite  occurs  as  an  essential  constit- 
uent of  certain  schistose  rocks  that  are  known  as  piedmontite  schists. 
It  occurs  also  in  veins  and  in  certain  volcanic  rocks,  where  it  is  probably 
an  alteration  product  of  feldspar.  Its  methods  of  origin  are  the  same 
as  those  of  epidote. 

Localities. — Good  crystals  are  found  in  the  manganese  ore  veins  at 
St.  Marcel,  Piedmont;  on  ilmenite  in  crystalline  schists  on  the  Isle  of 
Groix,  off  the  south  coast  of  Brittany;  and  at  a  number  of  points  on  the 
Island  of  Shikoku,  Japan,  in  crystalline  schists  and  in  ore  veins.  In 
the  United  States  it  is  so  abundant  in  the  acid  volcanic  rocks  of  South 
Mountain,  Penn.,  as  to  give  them  a  rose-red  color. 

Allanite  (Ca2(Al-Ce-Fe)3(OH)(Si04)3) 

Allanite  is  a  comparatively  rare  epidote  in  which  there  are  present 
notable  quantities  of  Ce,  Y,  La,  Di,  Er  and  occasionally  other  of  the 
rarer  elements.     Since  cerium  is  present  in  the  largest  quantity  the 


ANHYDROUS  ORTHOSILICATES 


331 


formula  of  the  mineral  is  usually  written  as  above,  with  the  under- 
standing that  a  portion  of  the  cerium  may  be  replaced  by  yttrium  and 
the  other  elements.  Some  idea  of  the  complex  character  of  the  mineral 
may  be  gained  from  the  two  analyses  quoted  below.  The  first  is  of 
crystals  from  Miask,  Ural,  and  the  second  of  a  black  massive  variety 
from  Douglas  Co.,  Colo. 


I 
30.81 
16.25 
6.29 


Si02 

AI2O3 

Fe203 

Ce2C>3 10.13 

BeO 

Di20a 3.43 

La20a 6.35 

Y2O3 1 .  24 

FeO 8.14 

MnO 2 .  25 

MgO 13 

CaO 1043 

K20 S3 

Na20 

H2O 2 .  79 

CO2 


Total 98.77 


II 

3i-i3 
11.44 

6.24 

12.50 

.27 

10.98 


13-59 
.61 

.16 

9-44 
tr. 

■56 

2.78 

.21 

99.81 


Allanite  rarely  occurs  in  crystals,  but  when  these  are  found  they  are 
usually  more  complex  than  those  of  piedmontite  but  much  less  compli- 
cated than  those  of  epidote.  Their  axial  ratio  is  1.5509  :  1  :  1.7691 
with  0=64°  59'.  Their  habit  is  like  that  of  epidote  crystals.  Common 
forms  are  00 P 06(100),  oP(ooi),  <»P(iio).  Twins  are  like  those  of 
epidote.  The  mineral  usually  occurs  as  massive,  granular  or  columnar 
aggregates,  or  as  ill-defined  columnar  crystals  resembling  rusty  nails. 
It  sometimes  forms  parallel  intergrowths  with  epidote. 

It  is  black  on  a  fresh  fracture  and  rusty  brown  on  exposed  surfaces, 
and  has  a  greenish  gray  or  brown  streak.  It  has  a  glassy  luster  and  is 
translucent  in  thin  splinters,  with  greenish  gray  or  brownish  tints  and 
is  pleochroic  in  various  shades  of  brown.  Its  hardness  is  5-6  and 
density  3-4,  both  varying  with  freshness  and  composition.  The  cleav- 
ages are  imperfect  and  the  fracture  uneven.  Its  indices  of  refraction 
are  nearly  the  same  as  those  of  epidote. 


32  DESCRIPTIVE  MINERALOGY 


Small  fragments  of  fresh  allanite  fuse  to  a  blebby  black  magnetic 
glass  before  the  blowpipe  and  are  decomposed  by  HC1  with  the  separa- 
tion of  gelatinous  silica. 

Allanite  is  distinguished  by  its  color,  manner  of  occurrence,  and  the 
reaction  for  water  in  the  closed  tube. 

The  mineral  alters  readily  on  exposure  to  the  weather,  yielding 
among  other  compounds  mica  and  limonite. 

Occurrence. — Allanite  occurs  as  an  original  constituent  in  some 
granites,  and  other  coarse-grained  rocks.  It  is  found  also  in  gneisses, 
occasionally  in  volcanic  rocks  and  rarely  as  a  metamorphic  mineral  in 
crystalline  limestones. 

Localities. — The  best  crystals  have  been  found  in  the  druses  of  a 
volcanic  rock  at  Lake  Laach,  Prussia;  in  coarse-grained  granitic  rocks 
at  several  places  in  the  Tyrol;  in  the  limestone  at  Pargas,  Finland;  and 
at  various  points  in  Ural,  Russia.  Massive  allanite  occurs  in  the  coarse 
granite  veins  at  Hittero,  Norway  and  as  the  constituents  of  granites 
at  many  places  in  the  United  States.  Parallel  intergrowths  with  epidote 
are  found  in  granite  at  Ilchester,  Md. 

CHONDRODITE  GROUP 

The  chondrocyte  group  of  minerals  includes  four  members  of  the 
general  formula  (Mg(F •  OH^Mg^SiC^)*  in  which  x  equals  i,  3,  5,  7,  and 
y,  1,  2,  3,  4.  Of  these,  one  (humite)  may  be  orthorhombic.  The  other 
three  are  monoclinic  with  the  angle  £=90°.  The  four  members  of  the 
group  with  their  compositions  and  axial  ratios  are: 

ProlectiU         (Mg(F-OH)2)Mg(Si04)      1.0803  :  1  :  1.8862  £=90° 
Chondrodite     (Mg(F-OH)2)Mg3(Si04)2  1.0863  :  1  :  3-1445  £=90 

b        Z 
Humite  (Mg(F-OH)2)Mg5(Si04)3  1.0802  :  1  :  4.4033 

Clinohumite    (Mg(F-OH)2)Mg7(Si04)4  1.0803  :  1  :  5.6588  0=90 

To  show  the  similarity  in  the  ratios  between  the  lateral  axes  of  the 
four  minerals,  the  &  axis  of  humite  is  written  as  1.  Chondrodite,  humite 
and  clinohumite  frequently  occur  together.  Chondrodite  has  been 
reported  at  more  localities  than  either  humite  or  clinohumite,  but  it  is 
not  certain  that  much  of  it  is  not  clinohumite.  The  three  minerals 
resemble  one  another  very  closely.  They  are  relatively  unstable  under 
conditions  prevailing  at  moderate  depths  in  the  earth's  crust,  passing 
easily  into  serpentine,  brucite  or  dolomite.  Only  chondrodite  is  de- 
scribed. 


ANHYDROUS  ORTHOSILICATES 


333 


Chondrodite  (Mg3(Mg(FOH)2)(Si04)2) 

Chondrodite  is  a  rather  uncommon  mineral  that  occurs  mainly  as  a 
constituent  of  metamorphosed  limestones  that  have  been  penetrated 
by  gases  and  water  emanating  from  igneous  rocks.  It  is  a  characteris- 
tic contact  mineral. 

Its  composition  varies  somewhat  in  consequence  of  the  fact  that  OH 
and  F  possess  the  power  to  mutually  replace  one  another.  The  two 
analyses  below  are  typical  of  varieties  containing  a  maximum  amount 
of  F. 


Si02 

MgO 

FeO 

H20 

I-  33  •  77 

57.98 

3-96 

i-37 

H.  35  42 

54-22 

5-72 

•   •   •   • 

F  F=0    Total 

5.14=102.22  —  2.16    100.06 
9 .  00=  104 .  36 — 3 .  78    100 .  58 

I.  Crystals  from  limestone  inclusions  in  the  lava  of  Vesuvius. 
II.  Grains  separated  from  the  limestone  of  the  Tilly  Foster  Iron  Mine,  Brewster, 

N.  Y. 

Chondrodite  is  monociinic  (prismatic  class),  with  an  axial  ratio 
1.0863  :  *  :  3I445-  0=9OO-  The  crystals  vary  widely  in  habit  and 
are  often  complex.  The  forms  oP(ooi), 
00  P  *  (100),  00  P  00  (010)  and  various  unit 
and  clinobemipyramids  of  the  general  sym- 
bol xPi  are  frequently  present,  but  other 
forms  are  also  common  (Fig.  181).  Twin- 
ning about  oP(ooi)  is  also  common. 
Usually,  however,  the  mineral  occurs  in 
little    rounded    grains,  in    some    instances 

showing    crystal    faces,    scattered    through  Fig.  181.— Chondrodite  Crys- 
tal with  oP,  001  (c);   JP  5b  , 

012  (0;  |F2,l27(fi);fP2, 
"5  (*);  fP2,  123  (r,); 
-2P2,  121  (r4);  -P,  in 
(*);  P,  In  (-»i);    JP 00, 

103  M;  P«,  101  (-*) 
and  -P5o,  101  («j).  The 
a  axis  runs  from  right  to  left 
and  the  upper  left  hand 
octant  is  assumed  to  be 
minus. 


limestone. 

When  fresh,  chondrodite  has  a  glassy 
luster,  is  translucent  and  is  white  or  has  a 
light  or  dark  yellow,  brown  or  garnet  color. 
It  has  a  distinct  cleavage  parallel  to  oP(ooi), 
a  conchoidal  fracture,  a  hardness  of  6  and 
a  density  of  3.15.  Its  refractive  indices 
for  yellow  light  are:  a=  1.607,  0=1.619, 
7=1.639. 

Before  the  blowpipe  chondrodite  bleaches 
without   fusing.     With  acids  it  decomposes  with  the  production  of 
gelatinous  silica. 


334 


DESCRIPTIVE  MINERALOGY 


It  weathers  readily  to  serpentine,  chlorite  and  brucite,  and  conse- 
quently many  grains  are  colored  dark  green  or  black. 

Occurrence. — Chondrodite,  as  has  been  stated,  occurs  in  meta- 
morphosed limestones.  It  also  occurs  in  sulphide  ore  bodies  and  in  a 
few  instances  in  magnetite  deposits.  It  is  probably  in  all  cases  a  pneu- 
matolytic  or  metamorphic  product. 

Localities. — It  is  found  as  crystals  in  the  blocks  enclosed  in  the  lavas 
of  Vesuvius;  in  the  copper  mines  of  Kapveltorp,  Sweden;  in  limestone 
in  the  Parish  of  Pargas,  Finland;  and  at  the  Tilly  Foster  Iron  Mine,  at 
Brewster,  N.  Y.  It  occurs  as  grains  in  the  crystalline  limestone  of 
Sussex  Co.,  N.  J.,  and  Orange  Co.,  N.  Y. 


DATOLITE  GROUP 

The  members  of  the  datolite  group  are  four  in  number,  but 
of  these  only  two,  viz.,  datolite  (Ca(B'OH)Si04)  and  gadolinite 
(Be2Fe(YO)2(Si0.i)2)  are  of  sufficient  importance  to  be  described  here. 
Both  minerals  crystallize  similarly  in  the  monoclinic  system  (mono- 
clinic  prismatic  class),  with  axial  ratios  that  are  nearly  alike. 

Datolite      alb:  ^=.6345  :  1  :  1.2657     /S  =  8o°5i' 
Gadolinite  alb:  £  =  .6273  :  1  :  1.3215    /S  =  8o°26i' 

Datolite  (Ca(BOH)Si04) 

Datolite,  or  datholite,  is  characteristically  a  vein  mineral. 

The  composition  corresponding  to  the 
formula  given  above  is: 

^       SiO=37.S4;    B20:*  =  21.83;    CaO=35.oo; 

1120=5.63.    Total=  100.00 


Some  specimens  contain  a  little  AI2O3  and 
Fe203  but,  in  general,  crystals  that  have 
been  analyzed  give  results  that  are  in 
close  accord  with  the  theoretical  com- 

Fig.   182 —Datolite  Crystal  with   P°sltl0n- 

oop 56,  100  (a);  00 p,  no  (m);        The  mineral  crystallizes  in  fine  crys- 

-P  co ,  101,  (<*>);  — JP  co ,  102  tals  that  are  often  very  complicated  (Fig. 

(*);   -P,  in  (»);  -P2,  212   !82).    About    115   different   forms  have 

(*);  P  * ,  on  (mz)  and  JP  <2 ,  been  observed  on  them.     Because  of  the 

suppression  of  some  faces  by  irregular 
growth  many  of  the  crystals  are  columnar  in  habit,  others  are  tabular. 
Most    crystals,    however,    are    nearly    equi-dimensional.     The    angle 


ANHYDROUS  ORTHOSILICATES  335 

noA  110=64°  40'.  The  mineral  occurs  also  in  globular,  radiating, 
granular  and  massive  forms. 

Datolite  is  colorless  or  white,  when  pure,  and  transparent.  Often, 
however,  it  is  greenish,  yellow,  reddish  or  violet,  and  translucent.  Its 
streak  is  white  and  its  luster  glassy.  It  has  no  distinct  cleavage.  Its 
fracture  is  conchoidal.  Its  hardness  is  5  and  its  sp.  gr.  about  3.  Some 
crystals  are  pyroelectric.  For  yellow  light,  0=1.6246,  18=1.6527, 
7=1.6694. 

Before  the  blowpipe  it  swells,  and  finally  melts  to  a  clear  glass  and, 
at  the  same  time,  it  colors  the  flame  green.  Its  powder  before  heating 
reacts  strongly  alkaline.  After  heating  this  reaction  is  weaker.  The 
mineral  loses  water  when  strongly  heated,  and  yields  gelatinous  silica 
when  treated  with  hydrochloric  acid. 

The  mineral  is  characterized  by  its  crystallization,  its  easy  fusibility 
and  the  flame  reaction  for  boron. 

Synthesis. — Datolite  has  not  been  produced  artificially. 

Occurrence,  Origin  and  Localities. — It  occurs  on  the  walls  of  clefts 
in  igneous  rocks,  in  pegmatite  veins  and  associated  with  metallic  com- 
pounds in  ore  veins.  It  is  found  in  many  ore  deposits  of  pneumatolytic 
origin,  notably  at  Andreasberg  in  the  Harz  Mts.;  at  Markirch,  in 
Alsace;  in  the  Seisser  Alps,  in  Tyrol;  in  the  Serra  dei  Zanchetti  in  the 
Bolognese  Apennines;  at  Arendal,  Norway,  and  at  many  other  places. 
In  North  America  it  occurs  at  Deerfield,  Mass.;  at  Tariffville,  Conn.; 
at  Bergen  Hill,  N.  J.;  and  at  several  points  in  the  copper  districts  of 
the  Lake  Superior  region. 

Gadolinite  (Be2Fe(YO)2(Si04)2) 

Gadolinite  is  a  rather  rare  mineral  with  a  composition  that  is  not 
well  established.  Its  occurrence  is  limited  to  coarse  granite  veins  or 
dikes — pegmatites — of  which  it  is  sometimes  a  constituent. 

Its  theoretical  composition  is  as  follows,  on  the  assumption  that  it  is 
analogous  to  that  of  datolite: 

SiO=  25.56;  ¥203=48.44;  FeO=  15.32;  BeO=  10.68.  Total=ioo.oo, 
but  nearly  all  specimens  contain  cerium  oxides.  Others  contain  nota- 
ble quantities  of  erbium  or  lanthanum  oxides  and  small  quantities  of 
thorium  oxide.  Nearly  all  show  the  presence  of  Fe203,  AI2O3,  CaO  and 
MgO,  and  in  some  helium  has  been  found. 

The  mineral  is  found  massive  and  in  rough  crystals  with  an  axial 
ratio  alb:  £=.6273  :  1  :  1.3215.  £=89°  263'.  The  crystals  show 
comparatively   few   forms,   of   which    oop(no),   oP(ooi),   Poo  (on), 


336  DESCRIPTIVE  MINERALOGY 

£P 00(012),  P(Tii)  and  —  P(in)  are  the  most  common.  They  are 
usually  columnar  in  habit  and  are  rough  and  coarse.  The  angle 
iioAiTo=64°  12'. 

Gadolinite  is  usually  black  or  greenish  black  and  opaque  or  trans- 
lucent, but  very  thin  splinters  of  fresh  specimens  are  translucent  or 
transparent  in  green  tints.  Its  luster  is  glassy  or  resinous,  streak 
greenish  gray  and  fracture  conchoidal.  Its  hardness  is  6-7  and  its 
density  about  4-4.5.  Upon  heating  the  density  increases.  Many  crys- 
tals appear  to  be  made  up  of  isotropic  and  anisotropic  substance,  and 
some  to  consist  entirely  of  isotropic  matter.  This  phenomenon  has 
been  explained  in  a  number  of  different  ways,  but  no  one  is  entirely  satis- 
factory. In  general,  the  isotropic  material  is  believed  to  be  an  amor- 
phous alteration  form  of  the  anisotropic  variety.  It  may  be  changed 
into  the  anisotropic  form  by  heating. 

The  crystallized  gadolinite  swells  up  in  the  blowpipe  flame  without 
becoming  fused  and  retains  its  transparency.  The  amorphous  variety 
also  swells  without  melting,  but  yields  a  grayish  green  translucent  mass. 
The  mineral  phosphoresces  when  heated  to  a  temperature  between  that 
of  melting  zinc  and  silver.  After  phosphorescing  it  is  unattacked  by 
hydrochloric  acid.  Before  heating  it  gelatinizes  with  the  same  reagent. 
The  mineral  is  weakly  radioactive. 

Localities  and  Origin. — Gadolinite  occurs  in  the  pegmatites  of  Ytterby 
near  Stockholm,  and  of  Fahlun,  Sweden;  on  the  Island  of  Hittero,  in 
southern  Norway;  in  the  Radauthal,  in  Harz;  at  Barringer  Hill,  Llano 
Co.,  Texas,  as  nodular  masses  and  large  rough  crystals;  and  at  Devil's 
Head,  Douglas  Co.,  Colo.  In  the  last  locality  it  occurs  in  a  de- 
composed granite  as  a  black  isotropic  variety  with  a  very  complex 
composition.     Specimens  analyzed  as  follows: 


I  n 

SiC>2 22.13  21.86 

ThCfe 89  .81 

AI2O3 2.34  .54 

Fe203 1 .  13  3-59 

Ce203 1 1. 10  6.87 

(La-Di)203 21.23  19.10 

Y2O3 9-5°  12.63 

Er203 12.74  15.80 


i  n 

FeO 10.43  1136 

BeO 7.19  5.46 

CaO 34  .47 

H20 86  .74 

Other .60  .79 


Total 100.48      100.02 


It  has  apparently  in  some  cases  solidified  from  an  igneous  magma. 
In  others  it  is  of  pneumatolytic  origin. 


ANHYDROUS  ORTHOSILICATES 


337 


Staurolite  (Fe(A10H)(A10)4(Si04)2) 

Staurolite  is  a  mineral  that  is  interesting  from  the  fact  that  it  fre- 
quently forms  twinned  crystals  that  resemble  a  cross  in  shape,  and  which 
consequently,  during  the  Middle  Ages,  was  held  in  great  veneration. 
Its  composition  is  not  well  established.  The  composition  indicated  by 
the  formula  above  is  as  shown  in  the  first  line  below  (I).  Three  analyses 
are  quoted  in  the  next  three  lines: 


Si02 

AI2O3 

Fe20s 

FeO 

MgO 

H20 

Ti02 

Total 

I. 

26.3 

55-9 

■   •   •   * 

158 

•   •   •   • 

2.00 

•   ■   ■ 

100.00 

II. 

27-38 

5420 

6.83 

913 

■   •   ■   • 

i-43 

•  •  • 

98.97 

III. 

3°-23 

51.16 

•   ■   •   • 

14.66 

2-73 

1.26 

.29 

100.33 

IV. 

27.91 

52.92 

6.87 

•    • 

7.80 

328 

i-59 

•  ■  • 

100.37 

I.  Theoretical  composition. 
II.  From  Monte  Campione,  Switzerland. 

III.  From  Morbihan,  France. 

IV.  From  Chesterfield,  Mass. 

Staurolite   crystallizes   in  the  orthorhombic  system   (bipyramidal 
class)  in  simple  crystals  with  the  axial  ratio  .4734  :  1  :  .6828.     The  indi- 


Fic.  183.  Fig.  184.  Fig.  185. 

Fig.  183. — Staurolite  Crystal  with  00 P,  no  (tn);    wPoo,  100  (6);  oP,  001  (c)  and 

Poo  ,  101  (r). 

Fig.  184. — Staurolite  Crystal  Twinned  about  fP  00  (032). 

Fig.  185. — Staurolite  Crystal  Twinned  about  |P|  (232). 

vidual  crystals  are  usually  bounded  by  00  P(i  10),  00  P  60  (001),  P  60  (101) 
and  often  oP(ooi),  but  all  their  faces  are  rough  (Fig.  183).  The  angle 
1 10 Alio  =50°  40'.  More  common,  however,  than  the  simple  crystals 
are  interpenetration  twins.  The  most  common  of  these  are  of  two  kinds, 
(1)  with  fP  06  (032)  the  twinning  plane  (Fig.  184),  and  (2)  with  ^£(232) 
the  twinning  plane  (Fig.  185).  Both  types  of  twins  yield  crosses,  but 
the  arms  of  the  first  type  are  perpendicular  to  one  another  and  those  of 


338  DESCRIPTIVE  MINERALOGY 

the  second  type  make  angles  of  about  6o°  and  1200.      Sometimes  the 
twinning  is  repeated,  giving  rise  to  trillings. 

The  mineral  is  reddish  or  blackish  brown,  and  has  a  glassy  or  greasy 
luster.  Its  streak  is  white.  It  is  slightly  translucent  in  fresh  crystals, 
but  usually  is  opaque.  In  very  thin  pieces  it  is  pleochroic  in  hyacinth- 
red  and  golden  yellow  tints.  Its  cleavage  is  distinct  parallel  to  00  P  06 
(010)  and  indistinct  parallel  to  ooP(no).  Its  fracture  is  conchoidal, 
its  hardness  7  and  its  density  3.4-3-8.  For  yellow  light,  a=  1.736, 
18=1.741,  7=1.746. 

Before  the  blowpipe  staurolite  is  infusible,  unless  it  contains  man- 
ganese, in  which  case  it  fuses  to  a  black  magnetic  glass.  It  is  only 
slightly  attacked  by  sulphuric  acid. 

It  is  distinguished  from  other  minerals  by  its  crystallization,  in- 
fusibility  and  hardness. 

Staurolite  weathers  fairly  readily  into  micaceous  minerals,  such  as 
chlorite  (p.  428)  and  muscovite  (p.  355). 

Synthesis. — It  has  not  been  produced  in  the  laboratory. 

Occurrence. — The  mineral  occurs  principally  in  mica  schist  and  other 
schistose  rocks  where  it  is  the  result  of  regional  or  contact  metamor- 
phism.  Because  of  its  method  of  occurrence  it  frequently  contains 
numerous  mineral  inclusions,  among  them  garnet  and  mica. 

Localities. — Good  crystals  of  staurolite  are  found  in  the  schists  at 
Mte.  Campione,  Switzerland;  in  the  Zillerthal,  Tyrof;  at  AschafFen- 
burg,  in  Bavaria;  at  various  places  in  Brittany,  France;  and  in  the 
United  States,  at  Windham,  Maine,  at  Franconia,  N.  H.,  at  Chester- 
field, Mass.,  in  Patrick  Co.,  Va.,  and  in  Fannin  Co.,  N.  C. 

Uses. — Twins  of  staurolite  are  used,  to  a  slight  extent,  as  jewelry. 
Specimens  from  Patrick  Co.,  Virginia,  are  mounted  and  worn  as  charms 
under  the  name  of  "  Fairy  Stones." 

Dumortierite  (Al(A10)7H(BO)(Si0.4)3) 

Dumortierite  is  one  of  the  few  blue  silicates  known.  It  is  a  borosili- 
cate  with  a  composition  approaching  the  formula  indicated  above.  The 
analysis  of  a  sample  from  Clip,  Arizona,  gave  (I) : 

Si02     AI2O3  Fe203  Ti203      MgO   B2O3    P2O5  Loss  on  Ign.  Total 

I.27.99    64.49     •••  tr         4.95     .20       1.72         99.35 

II.28.58    63.31     .21       1.49       5.21     ...        1.53        100.33 

Specimens  from  California  (II)  contain  in  addition  notable  quantities 
of  Ti02,  which  is  thought  to  exist  as  Ti203  replacing  a  part  of  the  AI2O3. 


ANHYDROUS  ORTHOSILICATES  339 

The  mineral  crystallizes  in  the  orthorhombic  system  in  aggregates  of 
fibers,  needles  or  very  thin  prisms  exhibiting  only  ooP(no)  and 
oo  P  56  (ioo)  without  end  faces.  Its  axial  ratio  is  a  :  £=.5317  '•  1,  and 
the  prismatic  angle  no  A  1^0=56°.  Its  crystals  possess  a  distinct 
cleavage  parallel  to  00  P  60  (100)  and  a  fracture  perpendicular  to  the 
long  axes  of  the  prisms.  Twinning  is  common,  with  ooP(no)  the 
twinning  plane. 

Dumortierite  is  commonly  some  shade  of  blue,  but  in  some  cases  is 
green,  lavender,  white,  or  colorless.  It  is  translucent  or  transparent 
and  strongly  pleochroic,  being  colorless  and  red,  purple  or  blue.  Its 
streak  is  light  blue.  Hardness  is  7  and  density  3.3.  Its  refractive  indices 
for  yellow  light  are:  a=  1.678,  /3=  1.686,  7=  1.089. 

Before  the  blowpipe  the  mineral  loses  its  color  and  is  infusible.  It  is 
insoluble  in  acids. 

It  is  distinguished  from  other  blue  silicates  by  its  fibrous  or  columnar 
character  and  its  insolubility  in  acids. 

Its  principal  alteration  products  are  kaolin  and  damourite 
(pp.  404,  357). 

Occurrence  and  Localities. — Dumortierite  occurs  only  as  a  constit- 
uent of  gneisses  and  pegmatites.  It  is  found  in  pegmatite  near  Lyons, 
France;  near  Schmiedeberg,  in  Silesia;  at  Harlem,  N.  Y.;  in  a  granular 
quartz,  at  Clip,  Yuma  Co.,  Ariz.,  and  in  a  dike  rock  composed  of  quartz 
and  dumortierite,  near  Dehesa,  San  Diego  Co.,  Cal.  It  is  evidently 
a  pneumatolytic  mineral.  Its  common  associates  are  kyanite,  anda- 
lusite  or  sillimanite. 

SODALITE  GROUP 

The  sodalite  group  includes  a  series  of  isometric  minerals  that  may  be 
regarded  as  compounds  of  silicates  with  a  sulphate,  a  sulphide  or  a  chlor- 
ide, or,  perhaps  better,  as  silicates  in  which  are  present  radicals  con- 
taining CI,  SO4  and  S.  The  minerals  comprising  the  group  are  haiiynite, 
nosean,  sodalite  and  lasurite.  Of  these,  sodalite  appears  to  be  a  mixture 
of  3NaAlSi04  and  NaCl,  in  which  the  CI  has  combined  with  one  atom  of 
Al,  thus  Na4(ClAl)Al2(Si04)3-  The  other  members  of  the  group  are 
comparable  with  this  on  the  assumption  that  the  CI  atom  is  replaced  by 
the  radicals  NaSC>4,  and  NaSa.  It  is  possible,  however,  that  all  are 
molecular  compounds  as  indicated  by  the  second  set  of  formulas  given 
below.  All  are  essentially  sodium  salts,  except  that  in  typical  haiiynite 
a  portion  of  the  Na  is  replaced  by  Ca.  The  chemical  symbols  of  the 
four  minerals  with  the  calculated  percentages  of  silica,  alumina  and 
soda  corresponding  to  their  formulas  are: 


340 


DESCRIPTIVE  MINERALOGY 


Si02 
37H 


AI2O3 
31.60 

27.03 

27.32 

26.9 


NasO 

25  .60 

27.26 

16.53 
27-3 


Sodalite    Nai(Cl •  Al)  AWSiQOs,  or 

3NaAlSi04-NaCl 
Noselite    Na4(NaS04  •  Al) Al2(Si04)3,  or  31 .  65 

3NaAlSi04  •  Na2S04 
Hatiynite  (Na2Ca)2(NaS04*  Al)Al2(SiOi)3,  or    31.99 

3NaAlSi04  •  CaS04 
Lasurite    Na4(NaS3  •  Al)Al2(Si04)3,  or  31.7 

3NaAlSi04-Na2S-S* 

SodaUte  (Na4(Cl-Al)Al2(Si04)3) 

Sodalite,  theoretically,  is  the  pure  sodium  compound  corresponding 
to  the  composition  indicated  by  the  formula  given  above.  Natural 
crystals,  however,  usually  contain  a  little  potassium  in  place  of  some  of 
the  sodium  and  often  some  calcium,  as  indicated  by  the  analyses  of 
material  from  Montreal,  Canada  (I),  and  Litchfield,  Maine  (II),  quoted 
below.    Moreover,  their  content  of  CI  is  not  constant. 


SiCfe    AI2O3  Na20  K2OCaO   CI  C1=0 

I-  37-52    31-38     25.15     .78    .35    6.91       =      102.09     -I-S5 

II-  37-33    31-87     24.56    .10    ...     6.83       -      101.76*   -1.54 

*  Includes  1.07  per  cent  H*0. 


Total 
100.54 
100.22 


Sodalite  occurs  massive  and  in  crystals  that  appear  to  be  holohedral, 
but  etch  figures  indicate  that  they  are  probably  tetrahedrally  hemi- 

hedral  (hextetrahedral  class).    Most  crystals 
are  dodecahedral  in  habit,  though  some  are 
tetrahexahedral  and  others  octahedral.    The 
forms     usually     developed    are     ooO(no), 
00O00  (100),  O(iii),  202(112)  and  404(114). 
Interpenetration  twins  of  two  dodecahedrons 
are  common,  with  O  the  twinning  plane  (Fig. 
186).    These  often  possess  an  hexagonal  habit. 
The  mineral  is  colorless,  white  or  some 
light  shade  of  blue  or  red,  and  its  streak  is 
white.    Its  luster  is  vitreous.    It   is   trans- 
parent,  translucent  and  sometimes  opaque. 
Its  cleavage  is  perfect  parallel  to  ooO(no) 
and  its  fracture  conchoidal.    Its  hardness  is  5-5.6,  and  its  density 
2.3.    Its  refractive  index  for  yellow  light,  n=  1.4827.    Some  specimens 
are  distinctly  fluorescent  and  phosphorescent. 


Fig.  186. — Sodalite.  Inter- 
penetration Twin  of  Two 
Dodecahedrons  Elon- 
gated in  the  Direction  of 
an  Octahedral  Axis  and 
Twinned  about  O(in). 


ANHYDROUS   ORTHOSILICATES  341 

Before  the  blowpipe,  colored  varieties  bleach  and  all  varieties  swell 
and  fuse  readily  to  a  colorless  blebby  glass.  The  mineral  dissolves  com- 
pletely in  strong  acids  and  yields  gelatinous  silica,  especially  after  heat- 
ing. When  dissolved  in  dilute  nitric  acid  its  solution  yields  a  chlorine 
precipitate  with  silver  nitrate.  Its  powder  becomes  brown  on  treatment 
with  AgNOa,  in  consequence  of  the  production  of  AgCl. 

The  mineral  is  best  distinguished  from  other  similarly  appearing 
minerals  by  the  production  of  gelatinous  silica  with  acids  and  the  reac- 
tion for  chlorine. 

As  a  result  of  weathering  sodalite  loses  CI  and  Na  and  gains  water. 
Its  commonest  alteration  products  are  zeolites  (p.  445),  kaolin  (p.  440), 
and  muscovite  (p.  355). 

Syntheses. — It  has  been  produced  artificially  by  dissolving  nepheline 
powder  in  fused  sodium  chloride,  and  by  decomposing  muscovite 
with  sodium  hydroxide  and  NaCl  at  a  temperature  of  5000  C. 

Occurrence  and  Origin. — Sodalite  occurs  principally  as  a  constituent 
of  igneous  rocks  rich  in  alkalies  and  as  crystals  on  the  walls  of  pores  in 
some  lavas.    It  is  also  known  as  an  alteration  product  of  nepheline. 

Localities. — Good  crystals  are  found  in  nepheline  syenite  at  Ditr6, 
in  Hungary,  in  the  lavas  of  Mte.  Somma,  Italy;  in  the  pegmatites  of 
southern  Norway;  and  at  many  other  points  where  nepheline  rocks 
occur.  In  North  America  it  is  abundant  in  the  rocks  at  Brome,  near 
Montreal;  in  the  Crazy  Mts.,  Montana,  and  at  Litchfield,  Maine.  The 
material  at  the  last-named  locality  is  light  blue. 

Noselite  and  Haiiynite  ((Na.Ca)2(NaS01Al)Al2(Si0.1)3) 

Noselite,  or  nosean,  and  haiiynite,  or  haiiyn,  consist  of  isomorphous 
mixtures  of  sodium  and  calcium  molecules  of  the  general  formula  given 
above.  Those  mixtures  containing  a  small  quantity  of  calcium  are 
usually  called  nosean,  while  those  with  larger  amounts  constitute  hatiyn. 
The  theoretical  nosean  and  haiiyn  molecules  are  indicated  on  p.  340. 
The  theoretical  compositions  of  the  pure  nosean  molecule  (I)  and  of  the 
most  common  hatiyn  mixture  (II)  are  as  follows: 

Si02      AI2O3  Fe203    CaO  Na20  Ka20  SO3  H20  Total 

27.26  ....  14.06  ....  100.00 

9.94  16.53  ....  14.22  ....  100.00 

.31         .21  20.91  ....  10.58  1.63  99.61* 

10.08  13.26  3.23  12.31  100.08 

*  Contains  also  .57  per  cent  CI. 


I. 

31-65 

27.03 

II. 

31  99 

2732 

III. 

35-99 

29.41 

IV. 

33.78 

27.42 

342  DESCRIPTIVE  MINERALOGY 

In  line  III  is  the  analysis  of  a  blue  nosean  from  Siderao,  Cape  Verde, 
and  in  line  IV,  the  analysis  of  a  blue  haiiyn  from  the  lava  of  Monte  Vul- 
ture, near  Melfi,  Italy. 

Nosean  and  haiiyn  are  isomorphous  with  sodalite.  They  crystallize 
is  the  isometric  system  in  simple  combinations  with  a  dodecahedral 
habit.  The  principal  forms  observed  are  ooO(no),  ooOoo(ioo) 
0002(102),  O(ni)  and  202(112).  Contact  and  interpenetration  twins 
are  common,  with  O(in)  the  twinning  plane.  The  twins  are  often 
columnar. 

The  minerals  have  a  glassy  or  greasy  luster,  are  transparent  or  trans- 
lucent, have  a  distinct  cleavage  parallel  to  ooO(no)  and  an  uneven  or 
conchoidal  fracture.  Their  hardness  is  5.6  and  density  2.25  to  2.5,  the 
value  increasing  with  the  amount  of  CaO  present.  Nosean  is  generally 
gray  and  haiiyn  blue,  but  both  minerals  may  possess  almost  any  color, 
from  white  through  light  green  and  blue  tints  to  black.  Red  colors  are 
rare.  The  streaks  of  both  minerals  are  colorless,  or  bluish.  For  yel- 
low, light  n=  1.4890  to  1.5038,  increasing  with  increase  in  the  Ca 
present.    Both  minerals  are  fluorescent  and  phosphorescent. 

Before  the  blowpipe  both  minerals  fuse  with  difficulty  to  a  blebby 
white  glass,  the  blue  haiiyn  retaining  its  color  until  a  high  temperature 
is  reached.  In  this  respect  it  differs  from  blue  sodalite  which  bleaches 
at  comparatively  low  temperatures.  Upon  treatment  with  hot  water 
both  minerals  yield  Na2S04.  They  are  decomposed  with  acids  yielding 
gelatinous  silica.  The  powders  of  both  minerals  react  alkaline.  Both 
also  give  the  sulphur  reaction  with  soda  on  charcoal. 

The  minerals  are  easily  distinguished  from  all  others  by  their  crys- 
tallization, gelatinization  with  acids  and  reaction  for  sulphur. 

Both  minerals  upon  weathering  yield  kaolin  or  zeolites  and 
calcite. 

Synthesis. — Crystals  of  noselite  have  been  made  by  melting  together 
Na2COa,  kaolinite  and  a  large  excess  of  Na2SO*. 

Occurrence. — Haiiyn  and  nosean  occur  in  many  rocks  containing 
nepheline,  especially  those  of  volcanic  origin  and  in  a  few  metamorphic 
rocks.  Haiiyn  is  so  common  in  some  of  them  as  to  constitute  an  essen- 
tial component. 

Localities. — Both  minerals  are  found  in  good  crystals  in  metamor- 
phosed inclusions  in  the  volcanic  rocks  of  the  Lake  Laach  region,  in 
Prussia;  also  in  the  rocks  of  the  Kaiserstuhl,  in  Baden;  in  those  of 
the  Albanian  Hills,  in  Italy,  and  at  S.  Antao  in  Cape  Verde.  In 
America  haiiyn  has  been  reported  from  the  nepheline  rocks  of  the 
Crazy  Mts.,  Montana. 


ANHYDROUS  ORTHOSILICATES  343 

Lasurite  (Na^NaSa-  Al)Al2(Si04)3) 

Lasurite  is  better  known  as  lapis  lazuli.  It  is  bright  blue  in  color 
and  was  formerly  much  used  as  a  gem  stone.  The  material  utilized  for 
gem  purposes  is  usually  a  mixture  of  different  minerals,  but  its  blue 
color  is  given  it  by  a  substance  with  a  composition  corresponding  to  the 
formula  indicated  above.  Since  the  artificial  ultramarine,  which  is 
ground  and  used  as  a  pigment,  also  has  this  composition,  the  molecule  is 
sometimes  represented  by  the  shortened  symbol  US3,  or  if  it  contains 
but  two  atoms  of  S,  by  the  symbol  US2.  The  deep  blue  lasurite  from 
Asia  contains  as  its  coloring  material  a  substance  with  a  composition 
that  may  be  represented  by  15.7  molecules  of  US3,  76.9  molecules  of 
hatiyn  and  7.4  molecules  of  sodalite,  corresponding  to  the  percentages: 


SiCb 

AI2O3 

CaO 

Na20              K20 

32-52  • 

27.61 

6.47 

19.45               .28 

SOa 

S 

CI 

Total  (Less  CI  =  0) 

10.46 

2.71 

•47 

99.97         =  99.42 

Lasurite  is  thus  the  name  given  to  the  blue  coloring  matter  of  lapis 
lazuli,  which  is  a  mixture.  It  apparently  crystallizes  in  dodecahedrons. 
Its  streak  is  blue,  its  cleavage  is  dodecahedral,  its  hardness  about  5  and 
its  specific  gravity  about  2.4.  Before  the  blowpipe  it  fuses  to  a  white 
glass.  Its  powder  bleaches  rapidly  in  hydrochloric  acid,  decomposes 
with  the  production  of  gelatinous  silica  and  yields  H2S. 

It  is  distinguished  from  blue  sodalite  and  hatiyn  by  the  reaction  with 
HC1,  especially  by  the  evolution  of  H2S. 

Occurrence. — Lasurite  is  principally  a  contact  mineral  in  limestone. 

Localities. — Good  lapis  lazuli  occurs  at  the  end  of  Lake  Baikal,  in 
Siberia;  in  the  Andes  of  Ovalle,  in  Chile;  in  the  limestone  inclusions  in 
the  lavas  of  Vesuvius,  and  in  the  Albanian  Mts.,  Italy. 

Uses. — Lapis  lazuli  is  used  as  an  ornamental  stone  in  the  manufacture 
of  vases,  and  various  ornaments,  in  the  manufacture  of  mosaics,  and  as  a 
pigment,  when  ground,  under  the  name  ultramarine.  Most  of  the  ultra- 
marine at  present  in  use,  however,  is  artificially  prepared. 

ACID  ORTHOSILICATES 

Prehnite  Q^CazAfeCSiO^a) 

Prehnite  is  nearly  always  found  in  crystals,  though  it  occurs  also  in 
stalactitic  and  granular  masses. 

The  theoretical  composition  of  the  pure  mineral  is  Si02  =  43.69, 


344  DESCRIPTIVE  MINERALOGY 

Al203  =  24.78,  CaO=  27.16,  and  H20=4-37-     Most  crystals,  however, 
contain  small  quantities  of  Fe203  and  other  constituents. 

SiO,  A1,0,  Fe«Oa  FeO  CaO  MgO  HaO  Total 

Jordansmtthl,  Silesia 44. 12  26.00      .61    25.26  tr  4.91  100.90 

Cornwall,  Penn 42.40  20.88    5.54    27.02  tr  4.01      99-85 

Chlorastrolite,  Isle  Royale  3741  24.62  2.21    1.81  22.20  3.46  7.72  99-75* 

*  Also  .32  per  cent  NajO. 

Its  crystallization  is  orthorhombic  and  hemimorphic  (rhombic  py- 
ramidal class),  with  a:  b:  c=. 8420:  1  :  1.1272.  The  crystals  vary 
widely  in  habit,  but  they  contain  comparatively  few  forms.  The  most 
prominent  are  oP(ooi),    ooP(no),  6Poo(o6i),  2P(22i)  and    6P(66i) 

(Fig.  187).  The  angle  noAiTo=8o° 
12'.  Because  they  exhibit  pyroelectric 
polarity  in  the  direction  of  the  a  axis  the 

,     .  ,       crvstals  are  thought  to  be  twins,  with 
Fig.  187. — Prehnite  Crystal  with  "t»  -  /       \  .1        A    •      •  1 

'       ,   ,.       n  -  /  x        00  P  00  (100)    as    the    twinning    plane. 

00  P,  no  (m);  00  poo  ,  100  (a);  v        '  &     v 

iP«,  304  (»);   iP*,  308  (»)      Cleavage  is  good  parallel  to  oP(ooi). 
and  oP,  001  (c).  The  crystals  are  frequently  tabular 

parallel  to  oP(ooi),  although  other 
habits  are  also  common.  Isolated  individuals  are  rare,  usually  many 
are  grouped  together  into  knotty  or  warty  aggregates. 

Prehnite  is  colorless  or  light  green,  and  transparent  or  trans- 
lucent, and  it  has  a  colorless  streak.  Its  luster  is  pearly  on  oP(ooi)  but 
glassy  on  other  faces.  Its  fracture  is  uneven,  its  hardness  7+  and  its 
density  2.80-2.95.     For  yellow  light,  a=  1.616,  0=  1.626,  7  =  1.649. 

Before  the  blowpipe  prehnite  exfoliates,  bleaches  and  melts  to  a 
yellowish  enamel.  At  a  high  temperature  it  yields  water.  Its  powder  is 
strongly  alkaline.  It  is  partially  decomposed  by  strong  hydrochloric 
acid  with  the  production  of  pulverulent  silica.  After  fusion  it  dissolves 
readily  in  this  acid  yielding  gelatinous  silica. 

The  mineral  has  not  been  produced  artificially. 

Occurrence. — Prehnite  occurs  as  crystals  implanted  on  the  walls  of 
clefts  in  siliceous  rocks,  in  the  gas  cavities  in  lavas,  and  in  the  gangue  of 
certain  ores,  especially  copper  ores.  It  is  found  also  as  pseudomorphs 
after  analcite  (p.  458),  laumonite  (p.  451),  and  natrolite  (p.  454).  In 
all  cases  it  is  probably  a  secondary  product. 

Localities. — Fine  crystals  come  from  veins  at  Harzburg,  in  Thiiringia; 
at  Striegau  and  Jordansmiihl,  Silesia,  and  at  Fassa  and  other  places  in 
Tyrol.  Good  crystals  are  found  also  in  the  Campsie  Hills  in  Scotland. 
The  mineral  is  abundant  in  veins  with  copper  along  the  north  shore  of 


ANHYDROUS  ORTHOSILICATES  345 

Lake  Superior  and  on  Keweenaw  Point,  and  it  occurs  also  at  Farmington, 
Conn.;  Bergen  Hill,  N.  J. ;  and  Cornwall,  Penn. 

Uses. — The  mineral  known  as  chlorastrolite  is  probably  an  impure 
prehnite.  It  is  found  on  the  beaches  of  Isle  Royale  and  the  north  shore 
of  Lake  Superior  as  little  pebbles  composed  of  stellar  and  radial  bunches 
of  bluish  green  fibers.  The  pebbles  were  originally  the  fillings  of  gas 
cavities  in  old  lavas.  They  are  polished  and  used,  to  a  slight  extent,  as 
gem-stones.  About  $2,000  worth  were  sold  in  191 1  and  $350  worth  in 
1912. 

Arinite  (H(Ca-Fe-Mn)3Al2B(Si04)4) 

Axinite  is  especially  noteworthy  for  its  richness  in  crystal  forms. 
The  mineral  is  a  complicated  borosilicate  for  which  the  formula  given 
above  is  merely  suggestive.  Analyses  of  crystals  from  different  localities 
vary  so  widely  that  no  satisfactory  simple  formula  has  been  proposed 
for  the  mineral.     Four  recent  analyses  are  quoted  below: 

Kadauthal  Stricgau  Oisans  Cornwall 

SiCfe 39  26  42.02  41 .53  42.10 

B2O3 4-91  5.00  4.62  4.64 

AI2O3 1446  17-73  *7  °o  I7-40 

FeoOa 2.62  .93  3.90  3.06 

FeO 3.65  6.55  4.02  5.84 

MnO 2.80  6.52  3.79  4.63 

CaO 29.70  19.21  21.66  2°S3 

MgO 2 .  00  .38  .74  .66 

H2O 1.22  1.77  2.16  1.80 


Total....  100.62    100. 11    100.32    100.66 

Axinite  crystallizes  in  the  triclinic  system  (pinacoidal  class),  with 
a:  b  :  £=.4921  :  1  :  .4797  and  a=82°  54',  0=91°  52',  7=131°  32'. 
The  crystals  are  extremely  varied  in  habit  but  nearly  all  are  somewhat 
tabular  parallel  to  'P(iii),  ooP'(no)  or  00  'P(iTo).  About  45  forms 
have  been  observed.  In  addition  to  the  three  mentioned,  2'P'  06  (201), 
P'(ui),  /P(iFi),  2/P'  06  (021),  00  P  06  (010)  and  00  P  00  (100)  are  the  most 
frequently  met  with  (Figs.  188,  189).  The  plane  'P(iTi)  is  usually 
striated  parallel  to  its  intersection  with  00  'P(iTo).  The  angle  100  A 1T0 
=  15°  34'.  The  cleavage  is  indistinct  parallel  to  ooP'(no)  and  the 
crystals  are  strongly  pyro  electric. 

Axinite  is  brownish,  gray,  green,  bluish  or  pink,  and  is  strongly  pleo- 
chroic  in  pearl-gray,   olive-green  and   cinnamon-brown   tints.     It   is 


346 


DESCRIPTIVE  MINERALOGY 


transparent  or  translucent  and  has  a  glassy  luster  and  a  colorless  streak. 
Its  fracture  is  conchoidal  or  uneven.  It  is  brittle,  has  a  hardness  of  6-7 
and  a  density  of  3.3.     For  red  light,  a=  1.6720,  0=  1.6779,  7  =  1.6810. 

Axinite,  before  the  blowpipe,  exfoliates  and  fuses  to  a  dark  green 
glass  which  becomes  black  in  the  oxidizing  flame.  It  colors  the  flame 
green,  especially  upon  the  addition  of  KHSO4  and  CaF2  to  its  powder. 
Its  powder  reacts  alkaline.    It  is  only  slightly  attacked  by  acids.    After 


Fig.  188. 


Fig.  189. 


Fig.  188. — Axinite  Crystal  with  00 Poo,  100  (a);    2'P'oo,  201  (5);    00 p/f  no  (f»); 

00 ,'P,  1T0  (M);  P',  in  (*)  and  'P,  1T1  (r). 
Fig.  189. — Axinite  Crystal  with  M ,  m,  a,  r  and  s  as  in  Fig.   188.    Also  00  P  00  , 

010  (b);  2P' »  ,  021  (y);  ,P,  In  (e);  §^3,  132  (<?);  4,P*5,  241  (0);  3JP3,  T31  (F); 

00  /FX  x30  (w);  3'PX  131  (»)  and  4'Pl,  241  (d). 

fusion,  however,  it  dissolves  readily  with  the  production  of  gelatinous 
silica. 

The  mineral  is  easily  characterized  by  its  crystallization  and  the 
green  color  it  imparts  to  the  flame. 

It  has  not  been  produced  artificially. 

Pseudormorphs  of  chlorite  after  axinite  have  been  found  in  Dart- 
moor, England. 

Occurrence. — Axinite  crystals  occur  in  cracks  in  old  siliceous  rocks. 
It  is  found  also  in  ore  veins  and.  as  a  component  of  a  contact  rock  com- 
posed mainly  of  augite,  hornblende  and  quartz,  occurring  near  the 
peripheries  of  granite  and  diabase  masses.  It  was  formed  by  the  aid 
of  pneumatolytic  processes. 

Localities. — Excellent  crystals  of  axinite  are  found  at  Andreasberg 
and  other  places  in  the  Harz  Mts.;  near  Striegau,  in  Silesia;  near 
Poloma,  in  Hungary;  at  the  Piz  Valatscha,  in  Switzerland;  near  Vernis 
and  at  other  points  in  Dauphine,  France;  at  Botallak,  Cornwall,  Eng- 
land; at  Konigsberg,  Norway;  Nordmark,  Sweden;  Lake  Onega,  and 
Miask,  Russia;  at  Wales  in  Maine  and  at  South  Bethlehem,  Penn. 


ANHYDROUS  ORTHOSILICATES  347 


Dioptase  (H2CuSi04) 

Dioptase  is  especially  interesting  because  of  its  crystallization,  which 
is  rhombohedral  tetartohedral  (trigonal  rhombohedral  class).  Its  crys- 
tals are  columnar.    Their  axial  ratio  is  i  :  .5342.    They  are  usually 

bounded  by  00  P2(ii2o),  —  2R(o22i)  or  R(ioii)  and  —  —  —  -  (1341)  or 

4P4  r       -  *    r 

+ (3141)  (a  rhombohedron  of  the  third  order,  Fig.  190).    Besides 

4    J 

occurring  as  crystals  the  mineral  is  found  also 
massive  and  in  crystalline  aggregates. 

The  composition  expressed  by  the  formula 
given  above  is  Si02=38.i8;  CuO=  50.40; 
H20=  11.44,  which  is  approached  very  closely 
by  some  analyses.  The  same  composition  may 
be  expressed  by  CuSiOa'JfeO.  Indeed,  recent 
work  indicates  that  the  mineral  is  a  hydrated 
metasilicate  and  not  an  acid  orthosilicate.  Fig.  190.— Dioptase  Crys- 

Dioptase  has  an  emerald-green  or  blackish      **!  with  *P2»  x«°  and 

green  color,  a  glassy  luster  and  a  green  streak.      ~~     '  °221  ^''  "^  a 
Ta    .  A  A         ,  ,    .    -         Rhombohedron    o£    the 

It  is   transparent   or   translucent,   is    brittle      3rd  Qrder  Indicated  by 

and  its  fracture  is  uneven  or  conchoidal.    Its      striations. 

hardness  is  5  and  its  density  3.05.    It  is  weakly 

pleochroic  and  is  distinctly  pyroelectric.     For  yellow  light,  «=  1.6580, 

€=1.7079. 

Before  the  blowpipe  dioptase  turns  black  and  colors  the  flame  green. 
On  charcoal  it  turns  black  in  the  oxidizing  flame  and  red  in  the  reducing 
flame  without  fusing.  It  is  decomposed  by  acids  with  the  production  of 
gelatinous  silica. 

Synthesis. — Crystals  of  dioptase  have  been  made  by  allowing  mix- 
tures of  copper  nitrate  and  potassium  silicate  to  diffuse  through  a  sheet 
of  parchment  paper. 

Occurrence  and  Localities. — The  mineral  occurs  in  druses  on  quartz 
in  clefts  in  limestone,  and  in  gold-bearing  placers  in  the  Altyn-Tiibe  Mt. 
near  the  Altyn  Ssu  River,  in  Siberia;  in  crystals  on  wulfenite  and  cala- 
mine and  embedded  in  clay  near  R6zbanya,  Hungary;  with  quartz  and 
chrysocolla  in  the  Mindonli  Mine,  French  Congo;  in  copper  mines  at 
Capiapo,  Chile;  and  in  Peru;  at  the  Bon  Ton  Mines,  Graham  Co., 
Ariz.;  and  near  Riverside,  Pinal  Co.,  in  the  same  State.  In  the  Bon 
Ton  Mines  it  covers  the  walls  of  cavities  in  the  ore,  which  consists  of  a 
mixture  of  limonite  and  copper  oxides. 


348 


DESCRIPTIVE  MINERALOGY 


MICA  GROUP 

The  mica  group  comprises  a  series  of  silicates  that  are  characterized 
by  such  perfect  cleavages  that  extremely  thin  lamellae  may  be  split 
from  them  with  surfaces  that  are  perfectly  smooth.  The  lamellae  are 
elastic  and  in  this  respect  the  members  of  the  group  are  different  from 
other  minerals  that  possess  an  almost  equally  perfect  cleavage.  Some 
of  the  micas  are  of  great  economic  importance,  but  most  of  them  have 
found  little  use  in  the  arts. 

The  micas  may  be  divided  into  four  subgroups,  (i)  the  magnesium- 
iron  micas,  (2)  the  calcium  micas,  (3)  the  lithium-iron  micas,  and  (4) 
the  alkali  micas.  Of  the  latter  there  are  three  subdivisions,  (a)  the 
lithia  micas,  (b)  the  potash  micas,  and  (c)  the  soda  micas. 

All  the  micas  crystallize  in  the  monoclinic  system  (monoclinic  pris- 
matic class),  in  crystals  with  an  orthorhombic  or  hexagonal  habit. 

In  composition  the  micas  are  complex.  The  alkali  micas  are  ap- 
parently acid  orthosilicates  of  aluminium  and  an  alkali — the  potash 
mica  being  KH2Al3(Si04)3.  Other  alkali  micas  are  more  acid,  and 
some  of  the  magnesium-iron  micas  are  very  complex.  The  members 
with  the  best  established  compositions  are  apparently  salts  of  orthosilicic 

acid,  and,  hence,  the  entire  group  is  placed 
with  the  orthosilicates. 

All  the  micas  possess  also,  in  addition  to 
the  very  noticeable  cleavage  which  yields 
the  characteristically  thin  lamellae  that  are 
so  well  known,  other  planes  of  parting 
which  are  well  exhibited  by  the  rays  of 
the  percussion  figure  (Fig.  191).  The 
largest  ray — known  as  the  characteristic 
ray — is  always  parallel  to  the  clinopinacoid. 
In  some  micas  the  plane  of  the  optical 
axes  is  the  clinopinacoid  and  in  others  is 
perpendicular  thereto.  In  the  latter,  known 
as  micas  of  the  first  order,  the  plane  of 
the  axes  is  perpendicular  to  the  characteristic  ray  and  in  the  former, 
distinguished  as  micas  of  the  second  order,  it  is  parallel  to  this  ray. 

The  value  of  the  optical  angle  varies  widely.  In  the  magnesia  micas 
it  is  between  o°  and  150,  in  the  calcium  micas  between  roo°  and  1200, 
and  in  the  other  micas  between  550  and  750.  When  the  angle  becomes 
zero  the  mineral  is  apparently  uniaxial.  But  etch  figures  on  all  micas 
indicate  a  monoclinic  symmetry  (compare  Fig.  194). 


>* 


Fig.    191. — Percussion    Figure 
on    Basal    Plane   of    Mica. 
The  long  ray  is  parallel   to 
00  P  ob  (010). 


ANHYDROUS  ORTHOSILICATES 


349 


THE   MAGNESIUM-IRON   MICAS 

Biotite  ((K-H)2(Mg-Fe)2(Al-Fe)2(Si04)3) 

The  magnesium-iron  micas  are  usually  designated  as  biotite.    This 
group  includes  micas  of  both  orders  as  follows: 


ist  Order 
Anomite 


2d  Order 
Meroxene 
Lepidomelane 
Phlogopiie 


The  crystals  of  biotite  have  an  axial  ratio  .5774  :  1  :  3.2904  with 
0= 900.  They  are  usually  simple  combinations  of  oP(ooi),  00  P  00  (010), 
— JP(ii2)  and  P(Tn)  (Fig.  192).  Twins  are 
common,  with  the  twinning  plane  perpendic- 
ular to  oP(ooi).  The  composition  face  may 
be  the  same  as  the  twinning  plane  or  it  may  be 
oP(ooi)  (Fig.  193).  The  crystals  have  an 
hexagonal  habit,  the  angle  In  A  010  being 
6o°  2 2 J'.  The  mineral  also  occurs  in  flat 
scales  and  in  scaly  aggregates. 

The  color  of  biotites  varies  from  yellow, 
through  green  and  brown  to  black.    Pleochroism  is  strong  in  sections 
perpendicular  to  the  perfect  cleavage,  i.e.,  perpendicular  to  oP(ooi). 
The  streak  of  all  varieties  is  white.    Their  hardness  =2.5  and  density 
2-7~3«I>  depending  upon  composition.     The  refractive  indices  for  yellow 


Fig.  192. — Biotite 
with  oP,  001  (c)\ 
010  (b);  P,  In 
-IP,  112  (*). 


Crystal 
00P00, 

(ju)  and 


0 

6 

\          A         \ 

£ 

I        -           ) 

Fig.  193. — Biotite  Twinned  about  a  Plane  Perpendicular  to  oP  (001),  and  Parallel 
to  the  Edge  Between  oP(ooi)  and  —  2P(22i).  The  composition  plane  is 
oP(ooi).    Mica  law.    A  =  right  hand  twin,  B  and  C  =  left  hand  twins. 

light  in  a  light  brown  biotite  from  Vesuvius  are:  a=i.54i2,  0=  1.5745. 
They  are  higher  in  darker  varieties. 

Etch  figures  are  produced  by  the  action  of  hot  concentrated  sulphuric 
acid. 

Varieties  and  their  Localities. — Anomite  is  rare.  It  occurs  at  Green- 
wood Furnace,  Orange  Co.,  N.  Y.,  and  at  Lake  Baikal,  in  Siberia. 


350  DESCRIPTIVE  MINERALOGY 

Merozene  is  the  name  given  to  the  common  biotite  of  the  2d  order. 
It  occurs  in  particularly  fine  crystals  in  the  limestone  blocks  included 
in  the  lava  of  Mte.  Somma,  Naples,  Italy;  at  various  points  in  Switzer- 
land, Austria  and  Hungary;  and  at  many  other  points  abroad  and  in 
this  country. 

Lepidomelane  is  a  black  meroxene  characterized  by  the  presence 
in  it  of  large  quantities  of  ferric  iron.  It  is  essentially  a  magnesium-free 
biotite.  It  occurs  in  igneous  rocks,  especially  those  rich  in  alkalies. 
Two  of  its  best  known  occurrences  in  the  United  States  are  in  the  nephe- 
line  syenite  at  Litchfield,  Maine,  and  in  a  pegmatite  in  the  northern  part 
of  Baltimore,  Md. 

Phlogopite,  or  amber  mica,  is  the  nearly  pure  magnesium  biotite 
which  by  most  mineralogists  is  regarded  as  a  distinct  mineral,  partly 
because  in  nearly  all  cases  it  contains  fluorine.  Its  color  is  yellowish 
brown,  brownish  red,  brownish  yellow,  green  or  white.  Its  luster  is 
often  pearly,  and  it  frequently  exhibits  asterism  in  consequence  of  the 
presence  of  inclusions  of  acicular  crystals  of  rutile  or  tourmaline  arranged 
along  the  rays  of  the  pressure  figure.  Its  axial  angle  is  small,  increasing 
with  increase  of  iron.  Its  refractive  indices  are:  a=  1.562,  0=  1.606, 
7=1.606. 

Phlogopite  is  especially  characteristic  of  metamorphosed  limestones. 
It  occurs  abundantly  in  the  metamorphosed  limestones  around  Easton, 
Pa.;  at  Edwards,  St.  Lawrence  Co.,  N.  Y.,  and  at  South  Burgess, 
Ontario,  Canada.  It  is  also  found  as  a  pyrogenetic  mineral  in  certain 
basic  igneous  rocks. 

Typical  analyses  of  the  four  varieties  of  biotite  follow: 

I  II                 in  IV 

Si02 40.81  35.79  32.35  39.66 

Ti02 3-51  tr.  .56 

AI2O3 16.47  x3-7°  *7-47  17.00 

Fe2C>3 2.16  4.04  24.22  .27 

FeO 5.92  1709  13.  n  .20 

MnO .40  1.20            

CaO 1.48  

BaO .33  .62 

MgO 21.08  9.68               .89  26.49 

Na20 1.55  -45  7-°°  -6° 


ANHYDROUS  ORTHOSILICATES  351 


I  II  III  IV 

K2O 9.01  8.20  6.40  9.97 

H20— \  .90  \  .66 

H20+ )    2I9  3^6  I4"67  2.33 

F .10            2 .  24 

Total  

(lessO=F)   99.19  99 .91  100.83  99.66 

I.  Anomite  from  Greenwood  Furnace,  Orange  Co.,  N.  Y. 
II.  Meroxene  from  granite,  Butte,  Mont. 

III.  Lepidomelane  from  eleolite  syenite,  Litchfield,  Maine. 

IV.  Brown  phlogopite  from  Burgess,  Can. 

Before  the  blowpipe  the  dark,  ferruginous  varieties  fuse  easily  to  a 
black  glass;  the  lighter  colored  varieties  with  greater  difficulty  to  a 
yellow  glass.  Their  powder  reactions  are  strongly  alkaline.  The 
minerals  are  not  attacked  by  HC1  but  are  decomposed  by  strong 
H2SO4.    In  the  closed  tube  all  varieties  give  a  little  water. 

The  biotites  are  distinguished  from  all  other  minerals  except  the  other 
micas  by  perfect  cleavage  and  from  other  micas  by  their  color,  solubility 
in  strong  sulphuric  acid  and  pleochroism. 

The  commoner  alteration  products  of  biotite  are  a  hydrated  biotite, 
chlorite  (p.  428),  epidote,  sillimanite  and  magnetite,  if  the  mica  is 
ferriferous.  At  the  same  time  there  is  often  a  separation  of  quartz. 
Phlogopite  alters  to  a  hydrophlogopite  and  to  penninite  (p.  429),  and 
talc  (p.  401). 

Syntheses. — The  biotites  are  common  products  of  smelting  operations. 
They  have  been  made  by  fusing  silicates  of  the  proper  composition  with 
sodium  and  magnesium  fluorides. 

Occurrences  and  Origin. — The  biotites  are  common  constituents  of 
igneous  and  metamorphic  rocks  and  pegmatite  dikes.  They  also  are 
common  alteration  products  of  certain  silicates,  such  as  hornblende 
and  augite.  They  are  present  in  sedimentary  rocks  principally  as  the 
products  of  weathering. 

Uses. — Phlogopite  is  used  as  an  insulator  in  electrical  appliances 
and  to  a  less  extent  for  the  same  purposes  as  those  for  which  ground 
muscovite  is  employed.  No  "  amber  mica "  is  produced  in  the 
United  States.  Most  of  that  used  in  this  country  is  imported  from 
Canada. 


352  DESCRIPTIVE  MINERALOGY 

THE  CALCIUM   MICAS 

Margarite  (Ca(A10)2(A10H)2(Si04)2) 

Margarite,  the  calcium  mica,  is  like  biotite  in  the  habit  of  its  crys- 
tals, which,  however,  are  not  so  well  formed  as  these.  Usually  the  min- 
eral occurs  in  tabular  plates  with  hexagonal  outlines  but  without  side 
planes.    It  occurs  also  as  scaly  aggregates. 

Analyses  of  specimens  from  Gainsville,  Ga.  (I),  and  Peekskill,  N.  Y. 
(II),  gave: 


Si02 

AI2O3 

FeO 

MgO 

CaO 

Na20 

H20 

Total 

I.  31.72 

50  03 

•      •      m      m 

.12 

"•57 

2.26 

4.88 

100.58 

II-  32-73 

46.58 

512 

1. 00 

11.04 

•   •   •   • 

449 

100.96 

The  mineral  has  a  pearly  luster  on  its  basal  planes,  and  a  glassy  luster 
on  other  planes.  Its  color  is  white,  yellowish,  or  gray  and  its  streak 
white.  It  is  transparent  or  translucent.  Its  cleavage  is  not  as  perfect 
as  in  the  other  micas,  and  its  cleavage  plates  are  less  elastic.  Its  hard- 
ness varies  from  3  to  over  4  and  its  density  is  3.  It  is  a  mica  of  the 
first  order. 

Before  the  blowpipe  it  swells,  but  fuses  with  great  difficulty.  It 
gives  water  in  the  closed  tube  and  is  attacked  by  acids. 

Occurrence. — Margarite  is  associated  with  corundum.  It  is  also 
present  in  some  chlorite  schists.     In  all  cases  it  is  of  metamorphic  origin. 

Localities. — It  occurs  in  the  Zillerthal,  Tyrol;  at  Campo  Longo,  in 
Switzerland;  at  the  emery  localities  in  the  Grecian  Archipelago;  at 
the  emery  mines  near  'Chester,  Mass.;  in  schist  inclusions  in  mica 
diorite  at  Peekskill,  N.  Y.;  with  corundum  at  Village  Green,  Penn.; 
at  the  Cullakenee  Mine,  in  Clay  Co.,  N.  C;  and  at  corundum  local- 
ities in  Georgia,  Alabama  and  Virginia. 

THE   LITHIUM-IRON   MICAS 

Zinnwaldite  ((Li-  K  Na)3FeAl(Al(F-  OH))2Si5Oi6) 

The  principal  lithium-iron  mica,  zinnwaldite,  is  a  very  complex 
mixture  that  occurs  in  several  forms  so  well  characterized  that  they  have 
received  different  names.  All  of  them  contain  lithium,  iron  and  fluorine, 
but  in  such  different  proportions  that  it  has  not  been  possible  to  ascribe 
to  them  any  one  generally  acceptable  formula.  Some  of  the  most  im- 
portant of  these  varieties  have  compositions  corresponding  to  the  fol- 
lowing analyses: 


ANHYDROUS  ORTHOSILICATES  353 

i  n  m  iv 

Si02 4019  59-25  5*46  45-87 

AI2O3 22.79  12.57  16.22  22.50 

Fe203 I9-78            2.21  .66 

FeO .93  7.66  11. 61 

MnO 2.02            .06  1.75 

Na20 7 .  63                .95  .42 

K20 7.49  5  37  IO-65  10.46 

Li20 3.06  904  4-83  3.28 

F 3-99  7-32  744  7-94 


Total..  99.32    102. 11    102.71    105.48 


—0=F=..  9764     9905     99.60    102.15 

I.  Rabenglimmer  from  Altenberg,  Saxony.    Greenish  black  with  greenish  gray 

streak.    Sp.  gr. =3.146-3.190. 
II.  Polylithionite  from  Kangerluarsuk,  Greenland.    White  or  light  green  plates. 
Sp.  gr.  =  2.81. 

III.  Cryophyllite  from  Rockport,  Mass.    Strongly  pleochroic  green  and  brown- 

ish red  crystals.    Sp.  gr.  =  2.000.    Contains  also  .17  MgO  and  1.06  HjO. 

IV.  Zinnwaldite  from  Zinnwald,  Bohemia.  Plates,  white,  yellow  or  greenish 
gray.    Sp.  gr.  =  2.956-2.987.    Contains  also  .91  H20  and  .08  PiO* 

Zinnwaldite  occurs  in  crystals  with  an  axial  ratio  very  near  that 
of  biotite,  and  a  tabular  habit.  Twins  are  like  those  of  biotite  with 
ooP(no)  the  twinning  plane. 

It  has  a  pearly  luster,  is  of  many  colors,  particularly  violet,  gray, 
yellow,  brown  and  dark  green  and  is  strongly  pleochroic.  Its  streak  is 
light,  its  hardness  between  2  and  3  and  its  density  between  2.8  and  3.2. 
It  is  a  mica  of  the  second  order. 

Before  the  blowpipe  it  fuses  to  a  dark,  weakly  magnetic  bead.  It  is 
attacked  by  acids. 

Occurrence  and  Localities. — Zinnwaldite  is  found  in  certain  ore  veins, 
in  granites  containing  cassiterite,  and  in  pegmatites.  Its  origin  is  as- 
cribed to  pneumatolytic  processes.  Its  principal  occurrences  are  those 
referred  to  in  connection  with  the  analyses. 

THE  ALKALI  MICAS 

The  alkali  micas  include  those  in  which  the  principal  metallic  con- 
stituents besides  aluminium  are  lithium,  potassium  and  sodium.  All 
these  metals  are  present  in  each  of  the  recognized  varieties  of  the 
alkali  micas,  but  in  each  variety  one  of  them  predominates.  That  in 
which  lithium  is  prominent  is  known  as  lepidolite;  that  in  which  potas- 


354  DESCRIPTIVE  MINERALOGY 

sium  is  most  abundant  is  muscovite;  and  that  in  which  sodium  is 
most  prominent  is  paragoniie.  Muscovite  is  common.  Lepidolite  is 
abundant  in  a  few  places.  Paragonite  is  rare.  The  first  two  are  im- 
portant economically.  All  are  micas  of  the  first  order,  except  a  few 
lepidolites,  and  all  are  light  colored. 

Another  mica,  which  is  usually  regarded  as  a  distinct  variety  of 
muscovite,  or,  at  any  rate,  as  being  very  closely  related  to  the  mineral 
is  roscoelite.  In  this,  about  two-thirds  of  the  AI2O3  in  muscovite  is 
replaced  by  V2O3.  It  is  a  rare  green  mica  which  is  utilized  as  an  ore 
of  vanadium. 

Lepidolite  ((Li-K.Na)2((Al-Fe)OH)2(Si03)3) 

Lepidolite  occurs  almost  exclusively  as  aggregates  of  thin  plates 
with  hexagonal  outlines.  Crystals  are  so  poorly  developed  that  a  satis- 
factory axial  ratio  has  not  been  determined.  Its  variation  in  composi- 
tion is  indicated  by  the  analyses  of  white  and  purple  varieties  from 
American  localities. 

in  III  IV 

Si02 51S2  49-52  51-12  5125 

AI2O3 25 .  96  28 .  80  22 .  70  25 .  62 

Fe2C>3 .31  40  .80  .12 

FeO undet.  .24  .00 

MnO .20  .07  i-34*  05 

MgO .02  .02  .00 

CaO 16  .13  tr. 

Li20 4.90  3.87  5.12     .        4.31 

Na20 1.06  .13  2.28  1.94 

K2O 1 1. 01  8.82  10.60  10.65 

Rb20 3.73            

Cs20 .08            

F 5.80  5.18  6.38             7.06 

H2O .95  1.72  2.05              1.60 


Total 

(lessO=F)   99.45  100.53  99  74  99 -63 

I.  Lilac-purple  granular  lepidolite  from  Rumford,  Maine. 
II.  White  variety  from  Norway,  Maine. 

III.  Red-purple  variety  from  Tourmaline  Queen  Mine,  Pala,  Cal.    Contains 

also  .04  P2O6. 

IV.  White  variety  from  Pala,  Cal. 

*  Mn2Os. 


■■■«• 


ANHYDROUS  ORTHOSILICATES  355 

The  mineral  is  white,  rose  or  light  purple,  gray  or  greenish.  The 
rose  and  purple  varieties  contain  a  little  manganese.  The  streak 
of  all  lepidolites  is  white,  their  luster*  pearly,  their  hardness  2.5-4 
and  density  2.8-2.9.  The  refractive  indices  of  a  typical  variety  are: 
0=1.5975,  7=1.6047. 

Lepidolite  fuses  easily  to  a  white  enamel  and  at  the  same  time  colors 
the  flame  red.  It  is  difficultly  attacked  by  acids,  but  after  heating  is 
easily  decomposed. 

Cookeitc  from  Maine  and  California  is  probably  a  weathered  lepido- 
lite.   Its  analyses  correspond  to  the  formula,  Li (Al (011)2)3 (Si03)2. 

Occurrence, — The  mineral  occurs  principally  in  pegmatites  in  which 
rubellite  (p.  435),  and  other  bright-colored  tourmalines  exist  and  on 
the  borders  of  granite  masses  and  in  rocks  adjacent  to  them.  It  is 
often  zonally  intergrown  with  muscovite.  In  all  cases  it  is  probably  a 
pneumatolytic  product,  or,  at  least,  is  produced  by  the  aid  of  magmatic 
emanations. 

Localities. — The  mineral  occurs  in  nearly  all  districts  producing  tin, 
and  also  in  those  producing  gem  tourmaline.  Its  best  known  foreign 
localities  are  Jekaterinburg,  Russia;  Rozna,  Moravia;  Schnittenhofen, 
Bohemia;  and  Penig,  Saxony.  In  the  United  States  it  is  found  in  large 
quantities  at  Hebron,  Paris,  and  other  points  in  western  Maine;  in  the 
tin  mines  of  the  southern  Black  Hills,  South  Dakota,  and  in  the  tourma- 
line localities  in  the  neighborhood  of  Pala,  San  Diego  Co.,  Cal. 

Uses. — Lepidolite  is  utilized  to  a  slight  extent  in  the  manufacture  of 
lithium  compounds,  which  are  employed  in  the  preparation  of  lithia 
waters,  medicinal  compounds,  salts  used  in  photography  and  in  the 
manufacture  of  fireworks  and  storage  batteries. 

Muscovite  (H2(K-Na)Al3(Si04)3) 

Muscovite  is  one  of  the  most  common,  and  at  the  same  time  the 
most  important,  of  the  micas.  Because  of  its  transparency  it  is  em- 
ployed for  many  purposes  for  which  the  darker  biotite  is  not  suitable. 

While  predominantly  a  potash  mica,  nearly  all  muscovite  contains 
some  soda,  due  to  the  isomorphous  mixture  of  the  paragonite  molecule. 
Two  typical  analyses  are  quoted  below: 

SiQi     AI2O3  Fe»0,    FeO   MnO  MgO  CaO    Na«0    K20     HtO      F    Total 

I.    44-39    35-70     109     1.07      tr        ...      .10     2.41       9.77     5.88     .72    101.13 

II.    46.54    34-96     1-59     32     ••■        -4i     10.38     5.43  99.63 

I.  Broad  plates  of  muscovite  bordered  by  lepidolite,  Auburn,  Maine. 
II.  Greenish  muscovite,  Auburn,  Maine.    Total  less  O  =  F  is  100.83. 


356 


DESCRIPTIVE  MINERALOGY 


The  crystals  are  usually  tabular  and  frequently  orthorhombic  or 
hexagonal  in  habit,  though  the  etch  figures  on  their  basal  planes  reveal 
clearly  their  monoclinic  symmetry  (Fig.  194).  If  orientated  to  corre- 
spond with  crystals  of  biotite  their  axial  constants  are:  a  :  b  :  £=.5774 
:  1  :  3.3128,  18=89°  54',  and  their  principal  planes  oP(ooi),  00  P  00  (010) 
|Pob  (023),  4P(44i)  and  -2P(22i)  (Fig.  195). 

Twins  like  those  of  biotite  are  not  uncommon  in  some  localities. 

Muscovite  is  colorless  or  of  some  light  shade  of  green,  yellow  or  red. 
It  has  a  glassy  luster,  a  perfect  cleavage  parallel  to  the  base,  a  hardness 
of  2  and  a  density  of  2.76-3.1.  Pleochroism  is  marked  in  directions 
perpendicular  and  parallel  to  the  cleavage,  the  color  of  the  crystals, 
when  viewed  in  the  direction  perpendicular  to  the  cleavage  being  lighter 


Fig.  194.  Fig.  195. 

Fig.  194. — Etch  Figures  on  oP(ooi)  of  Muscovite,  Exhibiting  Monoclinic  Symmetry. 

Fig.  195. — Muscovite  Crystal  with  —  2P,  221  (M);   oP,  001  (c);    00  Poo,  010  (6), 

and  §Pob  ,  023  (e). 


than  when  viewed  parallel  to  the  cleavage.  The  optical  angle  is  com- 
paratively large  (s6°-76°),  in  this  respect  being  very  different  from  that 
of  biotite  which  is  small  (2°-22°).  The  mineral  is  a  nonconductor 
of  electricity  at  ordinary  temperatures  and  a  poor  conductor  of  heat. 
Its  refractive  indices  vary  somewhat  with  composition.  For  yellow  light 
intermediate  values  are  as  follows:  a=  1.5619,  0=  1.5947,  7=  1.6027. 

Before  the  blowpipe  thin  flakes  of  muscovite  fuse  on  their  edges  to  a 
gray  mass.  In  the  closed  tube  the  mineral  yields  water  which,  in